| Message ID | 87609-531187-curtm@phaethon (mailing list archive) |
|---|---|
| State | Superseded |
| Delegated to: | BPF |
| Headers | show |
| Series | [v4] bpf: core: fix shift-out-of-bounds in ___bpf_prog_run | expand |
| Context | Check | Description |
|---|---|---|
| netdev/tree_selection | success | Not a local patch |
On 6/5/21 8:01 AM, Kurt Manucredo wrote: > Syzbot detects a shift-out-of-bounds in ___bpf_prog_run() > kernel/bpf/core.c:1414:2. This is not enough. We need more information on why this happens so we can judge whether the patch indeed fixed the issue. > > I propose: In adjust_scalar_min_max_vals() move boundary check up to avoid > missing them and return with error when detected. > > Reported-and-tested-by: syzbot+bed360704c521841c85d@syzkaller.appspotmail.com > Signed-off-by: Kurt Manucredo <fuzzybritches0@gmail.com> > --- > > https://syzkaller.appspot.com/bug?id=edb51be4c9a320186328893287bb30d5eed09231 > > Changelog: > ---------- > v4 - Fix shift-out-of-bounds in adjust_scalar_min_max_vals. > Fix commit message. > v3 - Make it clearer what the fix is for. > v2 - Fix shift-out-of-bounds in ___bpf_prog_run() by adding boundary > check in check_alu_op() in verifier.c. > v1 - Fix shift-out-of-bounds in ___bpf_prog_run() by adding boundary > check in ___bpf_prog_run(). > > thanks > > kind regards > > Kurt > > kernel/bpf/verifier.c | 30 +++++++++--------------------- > 1 file changed, 9 insertions(+), 21 deletions(-) > > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c > index 94ba5163d4c5..ed0eecf20de5 100644 > --- a/kernel/bpf/verifier.c > +++ b/kernel/bpf/verifier.c > @@ -7510,6 +7510,15 @@ static int adjust_scalar_min_max_vals(struct bpf_verifier_env *env, > u32_min_val = src_reg.u32_min_value; > u32_max_val = src_reg.u32_max_value; > > + if ((opcode == BPF_LSH || opcode == BPF_RSH || opcode == BPF_ARSH) && > + umax_val >= insn_bitness) { > + /* Shifts greater than 31 or 63 are undefined. > + * This includes shifts by a negative number. > + */ > + verbose(env, "invalid shift %lld\n", umax_val); > + return -EINVAL; > + } I think your fix is good. I would like to move after the following code though: if (!src_known && opcode != BPF_ADD && opcode != BPF_SUB && opcode != BPF_AND) { __mark_reg_unknown(env, dst_reg); return 0; } > + > if (alu32) { > src_known = tnum_subreg_is_const(src_reg.var_off); > if ((src_known && > @@ -7592,39 +7601,18 @@ static int adjust_scalar_min_max_vals(struct bpf_verifier_env *env, > scalar_min_max_xor(dst_reg, &src_reg); > break; > case BPF_LSH: > - if (umax_val >= insn_bitness) { > - /* Shifts greater than 31 or 63 are undefined. > - * This includes shifts by a negative number. > - */ > - mark_reg_unknown(env, regs, insn->dst_reg); > - break; > - } I think this is what happens. For the above case, we simply marks the dst reg as unknown and didn't fail verification. So later on at runtime, the shift optimization will have wrong shift value (> 31/64). Please correct me if this is not right analysis. As I mentioned in the early please write detailed analysis in commit log. Please also add a test at tools/testing/selftests/bpf/verifier/. > if (alu32) > scalar32_min_max_lsh(dst_reg, &src_reg); > else > scalar_min_max_lsh(dst_reg, &src_reg); > break; > case BPF_RSH: > - if (umax_val >= insn_bitness) { > - /* Shifts greater than 31 or 63 are undefined. > - * This includes shifts by a negative number. > - */ > - mark_reg_unknown(env, regs, insn->dst_reg); > - break; > - } > if (alu32) > scalar32_min_max_rsh(dst_reg, &src_reg); > else > scalar_min_max_rsh(dst_reg, &src_reg); > break; > case BPF_ARSH: > - if (umax_val >= insn_bitness) { > - /* Shifts greater than 31 or 63 are undefined. > - * This includes shifts by a negative number. > - */ > - mark_reg_unknown(env, regs, insn->dst_reg); > - break; > - } > if (alu32) > scalar32_min_max_arsh(dst_reg, &src_reg); > else >
On Sat, Jun 5, 2021 at 10:55 AM Yonghong Song <yhs@fb.com> wrote: > > > > On 6/5/21 8:01 AM, Kurt Manucredo wrote: > > Syzbot detects a shift-out-of-bounds in ___bpf_prog_run() > > kernel/bpf/core.c:1414:2. > > This is not enough. We need more information on why this happens > so we can judge whether the patch indeed fixed the issue. > > > > > I propose: In adjust_scalar_min_max_vals() move boundary check up to avoid > > missing them and return with error when detected. > > > > Reported-and-tested-by: syzbot+bed360704c521841c85d@syzkaller.appspotmail.com > > Signed-off-by: Kurt Manucredo <fuzzybritches0@gmail.com> > > --- > > > > https://syzkaller.appspot.com/bug?id=edb51be4c9a320186328893287bb30d5eed09231 > > > > Changelog: > > ---------- > > v4 - Fix shift-out-of-bounds in adjust_scalar_min_max_vals. > > Fix commit message. > > v3 - Make it clearer what the fix is for. > > v2 - Fix shift-out-of-bounds in ___bpf_prog_run() by adding boundary > > check in check_alu_op() in verifier.c. > > v1 - Fix shift-out-of-bounds in ___bpf_prog_run() by adding boundary > > check in ___bpf_prog_run(). > > > > thanks > > > > kind regards > > > > Kurt > > > > kernel/bpf/verifier.c | 30 +++++++++--------------------- > > 1 file changed, 9 insertions(+), 21 deletions(-) > > > > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c > > index 94ba5163d4c5..ed0eecf20de5 100644 > > --- a/kernel/bpf/verifier.c > > +++ b/kernel/bpf/verifier.c > > @@ -7510,6 +7510,15 @@ static int adjust_scalar_min_max_vals(struct bpf_verifier_env *env, > > u32_min_val = src_reg.u32_min_value; > > u32_max_val = src_reg.u32_max_value; > > > > + if ((opcode == BPF_LSH || opcode == BPF_RSH || opcode == BPF_ARSH) && > > + umax_val >= insn_bitness) { > > + /* Shifts greater than 31 or 63 are undefined. > > + * This includes shifts by a negative number. > > + */ > > + verbose(env, "invalid shift %lld\n", umax_val); > > + return -EINVAL; > > + } > > I think your fix is good. I would like to move after I suspect such change will break valid programs that do shift by register. > the following code though: > > if (!src_known && > opcode != BPF_ADD && opcode != BPF_SUB && opcode != BPF_AND) { > __mark_reg_unknown(env, dst_reg); > return 0; > } > > > + > > if (alu32) { > > src_known = tnum_subreg_is_const(src_reg.var_off); > > if ((src_known && > > @@ -7592,39 +7601,18 @@ static int adjust_scalar_min_max_vals(struct bpf_verifier_env *env, > > scalar_min_max_xor(dst_reg, &src_reg); > > break; > > case BPF_LSH: > > - if (umax_val >= insn_bitness) { > > - /* Shifts greater than 31 or 63 are undefined. > > - * This includes shifts by a negative number. > > - */ > > - mark_reg_unknown(env, regs, insn->dst_reg); > > - break; > > - } > > I think this is what happens. For the above case, we simply > marks the dst reg as unknown and didn't fail verification. > So later on at runtime, the shift optimization will have wrong > shift value (> 31/64). Please correct me if this is not right > analysis. As I mentioned in the early please write detailed > analysis in commit log. The large shift is not wrong. It's just undefined. syzbot has to ignore such cases.
On 6/5/21 12:10 PM, Alexei Starovoitov wrote: > On Sat, Jun 5, 2021 at 10:55 AM Yonghong Song <yhs@fb.com> wrote: >> >> >> >> On 6/5/21 8:01 AM, Kurt Manucredo wrote: >>> Syzbot detects a shift-out-of-bounds in ___bpf_prog_run() >>> kernel/bpf/core.c:1414:2. >> >> This is not enough. We need more information on why this happens >> so we can judge whether the patch indeed fixed the issue. >> >>> >>> I propose: In adjust_scalar_min_max_vals() move boundary check up to avoid >>> missing them and return with error when detected. >>> >>> Reported-and-tested-by: syzbot+bed360704c521841c85d@syzkaller.appspotmail.com >>> Signed-off-by: Kurt Manucredo <fuzzybritches0@gmail.com> >>> --- >>> >>> https://syzkaller.appspot.com/bug?id=edb51be4c9a320186328893287bb30d5eed09231 >>> >>> Changelog: >>> ---------- >>> v4 - Fix shift-out-of-bounds in adjust_scalar_min_max_vals. >>> Fix commit message. >>> v3 - Make it clearer what the fix is for. >>> v2 - Fix shift-out-of-bounds in ___bpf_prog_run() by adding boundary >>> check in check_alu_op() in verifier.c. >>> v1 - Fix shift-out-of-bounds in ___bpf_prog_run() by adding boundary >>> check in ___bpf_prog_run(). >>> >>> thanks >>> >>> kind regards >>> >>> Kurt >>> >>> kernel/bpf/verifier.c | 30 +++++++++--------------------- >>> 1 file changed, 9 insertions(+), 21 deletions(-) >>> >>> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c >>> index 94ba5163d4c5..ed0eecf20de5 100644 >>> --- a/kernel/bpf/verifier.c >>> +++ b/kernel/bpf/verifier.c >>> @@ -7510,6 +7510,15 @@ static int adjust_scalar_min_max_vals(struct bpf_verifier_env *env, >>> u32_min_val = src_reg.u32_min_value; >>> u32_max_val = src_reg.u32_max_value; >>> >>> + if ((opcode == BPF_LSH || opcode == BPF_RSH || opcode == BPF_ARSH) && >>> + umax_val >= insn_bitness) { >>> + /* Shifts greater than 31 or 63 are undefined. >>> + * This includes shifts by a negative number. >>> + */ >>> + verbose(env, "invalid shift %lld\n", umax_val); >>> + return -EINVAL; >>> + } >> >> I think your fix is good. I would like to move after > > I suspect such change will break valid programs that do shift by register. Oh yes, you are correct. We should guard it with src_known. But this should be extremely rare with explicit shifting amount being greater than 31/64 and if it is the case, the compiler will has a warning. > >> the following code though: >> >> if (!src_known && >> opcode != BPF_ADD && opcode != BPF_SUB && opcode != BPF_AND) { >> __mark_reg_unknown(env, dst_reg); >> return 0; >> } >> >>> + >>> if (alu32) { >>> src_known = tnum_subreg_is_const(src_reg.var_off); >>> if ((src_known && >>> @@ -7592,39 +7601,18 @@ static int adjust_scalar_min_max_vals(struct bpf_verifier_env *env, >>> scalar_min_max_xor(dst_reg, &src_reg); >>> break; >>> case BPF_LSH: >>> - if (umax_val >= insn_bitness) { >>> - /* Shifts greater than 31 or 63 are undefined. >>> - * This includes shifts by a negative number. >>> - */ >>> - mark_reg_unknown(env, regs, insn->dst_reg); >>> - break; >>> - } >> >> I think this is what happens. For the above case, we simply >> marks the dst reg as unknown and didn't fail verification. >> So later on at runtime, the shift optimization will have wrong >> shift value (> 31/64). Please correct me if this is not right >> analysis. As I mentioned in the early please write detailed >> analysis in commit log. > > The large shift is not wrong. It's just undefined. > syzbot has to ignore such cases. Agree. This makes sense.
On Sat, 5 Jun 2021 10:55:25 -0700, Yonghong Song <yhs@fb.com> wrote: > > > > On 6/5/21 8:01 AM, Kurt Manucredo wrote: > > Syzbot detects a shift-out-of-bounds in ___bpf_prog_run() > > kernel/bpf/core.c:1414:2. > > This is not enough. We need more information on why this happens > so we can judge whether the patch indeed fixed the issue. > > > > > I propose: In adjust_scalar_min_max_vals() move boundary check up to avoid > > missing them and return with error when detected. > > > > Reported-and-tested-by: syzbot+bed360704c521841c85d@syzkaller.appspotmail.com > > Signed-off-by: Kurt Manucredo <fuzzybritches0@gmail.com> > > --- > > > > https://syzkaller.appspot.com/bug?id=edb51be4c9a320186328893287bb30d5eed09231 > > > > Changelog: > > ---------- > > v4 - Fix shift-out-of-bounds in adjust_scalar_min_max_vals. > > Fix commit message. > > v3 - Make it clearer what the fix is for. > > v2 - Fix shift-out-of-bounds in ___bpf_prog_run() by adding boundary > > check in check_alu_op() in verifier.c. > > v1 - Fix shift-out-of-bounds in ___bpf_prog_run() by adding boundary > > check in ___bpf_prog_run(). > > > > thanks > > > > kind regards > > > > Kurt > > > > kernel/bpf/verifier.c | 30 +++++++++--------------------- > > 1 file changed, 9 insertions(+), 21 deletions(-) > > > > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c > > index 94ba5163d4c5..ed0eecf20de5 100644 > > --- a/kernel/bpf/verifier.c > > +++ b/kernel/bpf/verifier.c > > @@ -7510,6 +7510,15 @@ static int adjust_scalar_min_max_vals(struct bpf_verifier_env *env, > > u32_min_val = src_reg.u32_min_value; > > u32_max_val = src_reg.u32_max_value; > > > > + if ((opcode == BPF_LSH || opcode == BPF_RSH || opcode == BPF_ARSH) && > > + umax_val >= insn_bitness) { > > + /* Shifts greater than 31 or 63 are undefined. > > + * This includes shifts by a negative number. > > + */ > > + verbose(env, "invalid shift %lldn", umax_val); > > + return -EINVAL; > > + } > > I think your fix is good. I would like to move after > the following code though: > > if (!src_known && > opcode != BPF_ADD && opcode != BPF_SUB && opcode != BPF_AND) { > __mark_reg_unknown(env, dst_reg); > return 0; > } > It can only be right before that code not after. That's the latest. In the case of the syzbot bug, opcode == BPF_LSH and !src_known. Therefore it needs to be before that block of code. > > + > > if (alu32) { > > src_known = tnum_subreg_is_const(src_reg.var_off); > > if ((src_known && > > @@ -7592,39 +7601,18 @@ static int adjust_scalar_min_max_vals(struct bpf_verifier_env *env, > > scalar_min_max_xor(dst_reg, &src_reg); > > break; > > case BPF_LSH: > > - if (umax_val >= insn_bitness) { > > - /* Shifts greater than 31 or 63 are undefined. > > - * This includes shifts by a negative number. > > - */ > > - mark_reg_unknown(env, regs, insn->dst_reg); > > - break; > > - } > > I think this is what happens. For the above case, we simply > marks the dst reg as unknown and didn't fail verification. > So later on at runtime, the shift optimization will have wrong > shift value (> 31/64). Please correct me if this is not right > analysis. As I mentioned in the early please write detailed > analysis in commit log. > Shouldn't the src reg be changed so that the shift-out-of-bounds can't occur, if return -EINVAL is not what we want here? Changing the dst reg might not help. If I look into kernel/bpf/core.c I can see: DST = DST OP SRC; > Please also add a test at tools/testing/selftests/bpf/verifier/. > I'm going to look into selftests, kind regards thanks, Kurt Manucredo
On Sat, 5 Jun 2021 14:39:57 -0700, Yonghong Song <yhs@fb.com> wrote: > > > > On 6/5/21 12:10 PM, Alexei Starovoitov wrote: > > On Sat, Jun 5, 2021 at 10:55 AM Yonghong Song <yhs@fb.com> wrote: > >> > >> > >> > >> On 6/5/21 8:01 AM, Kurt Manucredo wrote: > >>> Syzbot detects a shift-out-of-bounds in ___bpf_prog_run() > >>> kernel/bpf/core.c:1414:2. > >> > >> This is not enough. We need more information on why this happens > >> so we can judge whether the patch indeed fixed the issue. > >> > >>> > >>> I propose: In adjust_scalar_min_max_vals() move boundary check up to avoid > >>> missing them and return with error when detected. > >>> > >>> Reported-and-tested-by: syzbot+bed360704c521841c85d@syzkaller.appspotmail.com > >>> Signed-off-by: Kurt Manucredo <fuzzybritches0@gmail.com> > >>> --- > >>> > >>> https://syzkaller.appspot.com/bug?id=edb51be4c9a320186328893287bb30d5eed09231 > >>> > >>> Changelog: > >>> ---------- > >>> v4 - Fix shift-out-of-bounds in adjust_scalar_min_max_vals. > >>> Fix commit message. > >>> v3 - Make it clearer what the fix is for. > >>> v2 - Fix shift-out-of-bounds in ___bpf_prog_run() by adding boundary > >>> check in check_alu_op() in verifier.c. > >>> v1 - Fix shift-out-of-bounds in ___bpf_prog_run() by adding boundary > >>> check in ___bpf_prog_run(). > >>> > >>> thanks > >>> > >>> kind regards > >>> > >>> Kurt > >>> > >>> kernel/bpf/verifier.c | 30 +++++++++--------------------- > >>> 1 file changed, 9 insertions(+), 21 deletions(-) > >>> > >>> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c > >>> index 94ba5163d4c5..ed0eecf20de5 100644 > >>> --- a/kernel/bpf/verifier.c > >>> +++ b/kernel/bpf/verifier.c > >>> @@ -7510,6 +7510,15 @@ static int adjust_scalar_min_max_vals(struct bpf_verifier_env *env, > >>> u32_min_val = src_reg.u32_min_value; > >>> u32_max_val = src_reg.u32_max_value; > >>> > >>> + if ((opcode == BPF_LSH || opcode == BPF_RSH || opcode == BPF_ARSH) && > >>> + umax_val >= insn_bitness) { > >>> + /* Shifts greater than 31 or 63 are undefined. > >>> + * This includes shifts by a negative number. > >>> + */ > >>> + verbose(env, "invalid shift %lldn", umax_val); > >>> + return -EINVAL; > >>> + } > >> > >> I think your fix is good. I would like to move after > > > > I suspect such change will break valid programs that do shift by register. > > Oh yes, you are correct. We should guard it with src_known. > But this should be extremely rare with explicit shifting amount being > greater than 31/64 and if it is the case, the compiler will has a > warning. > > > > >> the following code though: > >> > >> if (!src_known && > >> opcode != BPF_ADD && opcode != BPF_SUB && opcode != BPF_AND) { > >> __mark_reg_unknown(env, dst_reg); > >> return 0; > >> } > >> > >>> + > >>> if (alu32) { > >>> src_known = tnum_subreg_is_const(src_reg.var_off); > >>> if ((src_known && > >>> @@ -7592,39 +7601,18 @@ static int adjust_scalar_min_max_vals(struct bpf_verifier_env *env, > >>> scalar_min_max_xor(dst_reg, &src_reg); > >>> break; > >>> case BPF_LSH: > >>> - if (umax_val >= insn_bitness) { > >>> - /* Shifts greater than 31 or 63 are undefined. > >>> - * This includes shifts by a negative number. > >>> - */ > >>> - mark_reg_unknown(env, regs, insn->dst_reg); > >>> - break; > >>> - } > >> > >> I think this is what happens. For the above case, we simply > >> marks the dst reg as unknown and didn't fail verification. > >> So later on at runtime, the shift optimization will have wrong > >> shift value (> 31/64). Please correct me if this is not right > >> analysis. As I mentioned in the early please write detailed > >> analysis in commit log. > > > > The large shift is not wrong. It's just undefined. > > syzbot has to ignore such cases. > > Agree. This makes sense. Thanks for your input. If you find I should look closer into this bug just let me know. I'd love to help. If not it's fine, too. :-) kind regards, Kurt Manucredo
On Sat, Jun 5, 2021 at 9:10 PM Alexei Starovoitov <alexei.starovoitov@gmail.com> wrote: > On Sat, Jun 5, 2021 at 10:55 AM Yonghong Song <yhs@fb.com> wrote: > > On 6/5/21 8:01 AM, Kurt Manucredo wrote: > > > Syzbot detects a shift-out-of-bounds in ___bpf_prog_run() > > > kernel/bpf/core.c:1414:2. > > > > This is not enough. We need more information on why this happens > > so we can judge whether the patch indeed fixed the issue. > > > > > > > > I propose: In adjust_scalar_min_max_vals() move boundary check up to avoid > > > missing them and return with error when detected. > > > > > > Reported-and-tested-by: syzbot+bed360704c521841c85d@syzkaller.appspotmail.com > > > Signed-off-by: Kurt Manucredo <fuzzybritches0@gmail.com> > > > --- > > > > > > https://syzkaller.appspot.com/bug?id=edb51be4c9a320186328893287bb30d5eed09231 > > > > > > Changelog: > > > ---------- > > > v4 - Fix shift-out-of-bounds in adjust_scalar_min_max_vals. > > > Fix commit message. > > > v3 - Make it clearer what the fix is for. > > > v2 - Fix shift-out-of-bounds in ___bpf_prog_run() by adding boundary > > > check in check_alu_op() in verifier.c. > > > v1 - Fix shift-out-of-bounds in ___bpf_prog_run() by adding boundary > > > check in ___bpf_prog_run(). > > > > > > thanks > > > > > > kind regards > > > > > > Kurt > > > > > > kernel/bpf/verifier.c | 30 +++++++++--------------------- > > > 1 file changed, 9 insertions(+), 21 deletions(-) > > > > > > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c > > > index 94ba5163d4c5..ed0eecf20de5 100644 > > > --- a/kernel/bpf/verifier.c > > > +++ b/kernel/bpf/verifier.c > > > @@ -7510,6 +7510,15 @@ static int adjust_scalar_min_max_vals(struct bpf_verifier_env *env, > > > u32_min_val = src_reg.u32_min_value; > > > u32_max_val = src_reg.u32_max_value; > > > > > > + if ((opcode == BPF_LSH || opcode == BPF_RSH || opcode == BPF_ARSH) && > > > + umax_val >= insn_bitness) { > > > + /* Shifts greater than 31 or 63 are undefined. > > > + * This includes shifts by a negative number. > > > + */ > > > + verbose(env, "invalid shift %lld\n", umax_val); > > > + return -EINVAL; > > > + } > > > > I think your fix is good. I would like to move after > > I suspect such change will break valid programs that do shift by register. > > > the following code though: > > > > if (!src_known && > > opcode != BPF_ADD && opcode != BPF_SUB && opcode != BPF_AND) { > > __mark_reg_unknown(env, dst_reg); > > return 0; > > } > > > > > + > > > if (alu32) { > > > src_known = tnum_subreg_is_const(src_reg.var_off); > > > if ((src_known && > > > @@ -7592,39 +7601,18 @@ static int adjust_scalar_min_max_vals(struct bpf_verifier_env *env, > > > scalar_min_max_xor(dst_reg, &src_reg); > > > break; > > > case BPF_LSH: > > > - if (umax_val >= insn_bitness) { > > > - /* Shifts greater than 31 or 63 are undefined. > > > - * This includes shifts by a negative number. > > > - */ > > > - mark_reg_unknown(env, regs, insn->dst_reg); > > > - break; > > > - } > > > > I think this is what happens. For the above case, we simply > > marks the dst reg as unknown and didn't fail verification. > > So later on at runtime, the shift optimization will have wrong > > shift value (> 31/64). Please correct me if this is not right > > analysis. As I mentioned in the early please write detailed > > analysis in commit log. > > The large shift is not wrong. It's just undefined. > syzbot has to ignore such cases. Hi Alexei, The report is produced by KUBSAN. I thought there was an agreement on cleaning up KUBSAN reports from the kernel (the subset enabled on syzbot at least). What exactly cases should KUBSAN ignore? +linux-hardening/kasan-dev for KUBSAN false positive
On Mon, Jun 07, 2021 at 09:38:43AM +0200, 'Dmitry Vyukov' via Clang Built Linux wrote: > On Sat, Jun 5, 2021 at 9:10 PM Alexei Starovoitov > <alexei.starovoitov@gmail.com> wrote: > > On Sat, Jun 5, 2021 at 10:55 AM Yonghong Song <yhs@fb.com> wrote: > > > On 6/5/21 8:01 AM, Kurt Manucredo wrote: > > > > Syzbot detects a shift-out-of-bounds in ___bpf_prog_run() > > > > kernel/bpf/core.c:1414:2. > > > > > > This is not enough. We need more information on why this happens > > > so we can judge whether the patch indeed fixed the issue. > > > > > > > > > > > I propose: In adjust_scalar_min_max_vals() move boundary check up to avoid > > > > missing them and return with error when detected. > > > > > > > > Reported-and-tested-by: syzbot+bed360704c521841c85d@syzkaller.appspotmail.com > > > > Signed-off-by: Kurt Manucredo <fuzzybritches0@gmail.com> > > > > --- > > > > > > > > https://syzkaller.appspot.com/bug?id=edb51be4c9a320186328893287bb30d5eed09231 > > > > > > > > Changelog: > > > > ---------- > > > > v4 - Fix shift-out-of-bounds in adjust_scalar_min_max_vals. > > > > Fix commit message. > > > > v3 - Make it clearer what the fix is for. > > > > v2 - Fix shift-out-of-bounds in ___bpf_prog_run() by adding boundary > > > > check in check_alu_op() in verifier.c. > > > > v1 - Fix shift-out-of-bounds in ___bpf_prog_run() by adding boundary > > > > check in ___bpf_prog_run(). > > > > > > > > thanks > > > > > > > > kind regards > > > > > > > > Kurt > > > > > > > > kernel/bpf/verifier.c | 30 +++++++++--------------------- > > > > 1 file changed, 9 insertions(+), 21 deletions(-) > > > > > > > > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c > > > > index 94ba5163d4c5..ed0eecf20de5 100644 > > > > --- a/kernel/bpf/verifier.c > > > > +++ b/kernel/bpf/verifier.c > > > > @@ -7510,6 +7510,15 @@ static int adjust_scalar_min_max_vals(struct bpf_verifier_env *env, > > > > u32_min_val = src_reg.u32_min_value; > > > > u32_max_val = src_reg.u32_max_value; > > > > > > > > + if ((opcode == BPF_LSH || opcode == BPF_RSH || opcode == BPF_ARSH) && > > > > + umax_val >= insn_bitness) { > > > > + /* Shifts greater than 31 or 63 are undefined. > > > > + * This includes shifts by a negative number. > > > > + */ > > > > + verbose(env, "invalid shift %lld\n", umax_val); > > > > + return -EINVAL; > > > > + } > > > > > > I think your fix is good. I would like to move after > > > > I suspect such change will break valid programs that do shift by register. > > > > > the following code though: > > > > > > if (!src_known && > > > opcode != BPF_ADD && opcode != BPF_SUB && opcode != BPF_AND) { > > > __mark_reg_unknown(env, dst_reg); > > > return 0; > > > } > > > > > > > + > > > > if (alu32) { > > > > src_known = tnum_subreg_is_const(src_reg.var_off); > > > > if ((src_known && > > > > @@ -7592,39 +7601,18 @@ static int adjust_scalar_min_max_vals(struct bpf_verifier_env *env, > > > > scalar_min_max_xor(dst_reg, &src_reg); > > > > break; > > > > case BPF_LSH: > > > > - if (umax_val >= insn_bitness) { > > > > - /* Shifts greater than 31 or 63 are undefined. > > > > - * This includes shifts by a negative number. > > > > - */ > > > > - mark_reg_unknown(env, regs, insn->dst_reg); > > > > - break; > > > > - } > > > > > > I think this is what happens. For the above case, we simply > > > marks the dst reg as unknown and didn't fail verification. > > > So later on at runtime, the shift optimization will have wrong > > > shift value (> 31/64). Please correct me if this is not right > > > analysis. As I mentioned in the early please write detailed > > > analysis in commit log. > > > > The large shift is not wrong. It's just undefined. > > syzbot has to ignore such cases. > > Hi Alexei, > > The report is produced by KUBSAN. I thought there was an agreement on > cleaning up KUBSAN reports from the kernel (the subset enabled on > syzbot at least). > What exactly cases should KUBSAN ignore? > +linux-hardening/kasan-dev for KUBSAN false positive Can check_shl_overflow() be used at all? Best to just make things readable and compiler-happy, whatever the implementation. :)
On 6/9/21 11:20 AM, Kees Cook wrote: > On Mon, Jun 07, 2021 at 09:38:43AM +0200, 'Dmitry Vyukov' via Clang Built Linux wrote: >> On Sat, Jun 5, 2021 at 9:10 PM Alexei Starovoitov >> <alexei.starovoitov@gmail.com> wrote: >>> On Sat, Jun 5, 2021 at 10:55 AM Yonghong Song <yhs@fb.com> wrote: >>>> On 6/5/21 8:01 AM, Kurt Manucredo wrote: >>>>> Syzbot detects a shift-out-of-bounds in ___bpf_prog_run() >>>>> kernel/bpf/core.c:1414:2. >>>> >>>> This is not enough. We need more information on why this happens >>>> so we can judge whether the patch indeed fixed the issue. >>>> >>>>> >>>>> I propose: In adjust_scalar_min_max_vals() move boundary check up to avoid >>>>> missing them and return with error when detected. >>>>> >>>>> Reported-and-tested-by: syzbot+bed360704c521841c85d@syzkaller.appspotmail.com >>>>> Signed-off-by: Kurt Manucredo <fuzzybritches0@gmail.com> >>>>> --- >>>>> >>>>> https://syzkaller.appspot.com/bug?id=edb51be4c9a320186328893287bb30d5eed09231 >>>>> >>>>> Changelog: >>>>> ---------- >>>>> v4 - Fix shift-out-of-bounds in adjust_scalar_min_max_vals. >>>>> Fix commit message. >>>>> v3 - Make it clearer what the fix is for. >>>>> v2 - Fix shift-out-of-bounds in ___bpf_prog_run() by adding boundary >>>>> check in check_alu_op() in verifier.c. >>>>> v1 - Fix shift-out-of-bounds in ___bpf_prog_run() by adding boundary >>>>> check in ___bpf_prog_run(). >>>>> >>>>> thanks >>>>> >>>>> kind regards >>>>> >>>>> Kurt >>>>> >>>>> kernel/bpf/verifier.c | 30 +++++++++--------------------- >>>>> 1 file changed, 9 insertions(+), 21 deletions(-) >>>>> >>>>> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c >>>>> index 94ba5163d4c5..ed0eecf20de5 100644 >>>>> --- a/kernel/bpf/verifier.c >>>>> +++ b/kernel/bpf/verifier.c >>>>> @@ -7510,6 +7510,15 @@ static int adjust_scalar_min_max_vals(struct bpf_verifier_env *env, >>>>> u32_min_val = src_reg.u32_min_value; >>>>> u32_max_val = src_reg.u32_max_value; >>>>> >>>>> + if ((opcode == BPF_LSH || opcode == BPF_RSH || opcode == BPF_ARSH) && >>>>> + umax_val >= insn_bitness) { >>>>> + /* Shifts greater than 31 or 63 are undefined. >>>>> + * This includes shifts by a negative number. >>>>> + */ >>>>> + verbose(env, "invalid shift %lld\n", umax_val); >>>>> + return -EINVAL; >>>>> + } >>>> >>>> I think your fix is good. I would like to move after >>> >>> I suspect such change will break valid programs that do shift by register. >>> >>>> the following code though: >>>> >>>> if (!src_known && >>>> opcode != BPF_ADD && opcode != BPF_SUB && opcode != BPF_AND) { >>>> __mark_reg_unknown(env, dst_reg); >>>> return 0; >>>> } >>>> >>>>> + >>>>> if (alu32) { >>>>> src_known = tnum_subreg_is_const(src_reg.var_off); >>>>> if ((src_known && >>>>> @@ -7592,39 +7601,18 @@ static int adjust_scalar_min_max_vals(struct bpf_verifier_env *env, >>>>> scalar_min_max_xor(dst_reg, &src_reg); >>>>> break; >>>>> case BPF_LSH: >>>>> - if (umax_val >= insn_bitness) { >>>>> - /* Shifts greater than 31 or 63 are undefined. >>>>> - * This includes shifts by a negative number. >>>>> - */ >>>>> - mark_reg_unknown(env, regs, insn->dst_reg); >>>>> - break; >>>>> - } >>>> >>>> I think this is what happens. For the above case, we simply >>>> marks the dst reg as unknown and didn't fail verification. >>>> So later on at runtime, the shift optimization will have wrong >>>> shift value (> 31/64). Please correct me if this is not right >>>> analysis. As I mentioned in the early please write detailed >>>> analysis in commit log. >>> >>> The large shift is not wrong. It's just undefined. >>> syzbot has to ignore such cases. >> >> Hi Alexei, >> >> The report is produced by KUBSAN. I thought there was an agreement on >> cleaning up KUBSAN reports from the kernel (the subset enabled on >> syzbot at least). >> What exactly cases should KUBSAN ignore? >> +linux-hardening/kasan-dev for KUBSAN false positive > > Can check_shl_overflow() be used at all? Best to just make things > readable and compiler-happy, whatever the implementation. :) This is not a compile issue. If the shift amount is a constant, compiler should have warned and user should fix the warning. This is because user code has something like a << s; where s is a unknown variable and verifier just marked the result of a << s as unknown value. Verifier may not reject the code depending on how a << s result is used. If bpf program writer uses check_shl_overflow() or some kind of checking for shift value and won't do shifting if the shifting may cause an undefined result, there should not be any kubsan warning. >
On Thu, Jun 10, 2021 at 1:40 AM Yonghong Song <yhs@fb.com> wrote: > On 6/9/21 11:20 AM, Kees Cook wrote: > > On Mon, Jun 07, 2021 at 09:38:43AM +0200, 'Dmitry Vyukov' via Clang Built Linux wrote: > >> On Sat, Jun 5, 2021 at 9:10 PM Alexei Starovoitov > >> <alexei.starovoitov@gmail.com> wrote: > >>> On Sat, Jun 5, 2021 at 10:55 AM Yonghong Song <yhs@fb.com> wrote: > >>>> On 6/5/21 8:01 AM, Kurt Manucredo wrote: > >>>>> Syzbot detects a shift-out-of-bounds in ___bpf_prog_run() > >>>>> kernel/bpf/core.c:1414:2. > >>>> > >>>> This is not enough. We need more information on why this happens > >>>> so we can judge whether the patch indeed fixed the issue. > >>>> > >>>>> > >>>>> I propose: In adjust_scalar_min_max_vals() move boundary check up to avoid > >>>>> missing them and return with error when detected. > >>>>> > >>>>> Reported-and-tested-by: syzbot+bed360704c521841c85d@syzkaller.appspotmail.com > >>>>> Signed-off-by: Kurt Manucredo <fuzzybritches0@gmail.com> > >>>>> --- > >>>>> > >>>>> https://syzkaller.appspot.com/bug?id=edb51be4c9a320186328893287bb30d5eed09231 > >>>>> > >>>>> Changelog: > >>>>> ---------- > >>>>> v4 - Fix shift-out-of-bounds in adjust_scalar_min_max_vals. > >>>>> Fix commit message. > >>>>> v3 - Make it clearer what the fix is for. > >>>>> v2 - Fix shift-out-of-bounds in ___bpf_prog_run() by adding boundary > >>>>> check in check_alu_op() in verifier.c. > >>>>> v1 - Fix shift-out-of-bounds in ___bpf_prog_run() by adding boundary > >>>>> check in ___bpf_prog_run(). > >>>>> > >>>>> thanks > >>>>> > >>>>> kind regards > >>>>> > >>>>> Kurt > >>>>> > >>>>> kernel/bpf/verifier.c | 30 +++++++++--------------------- > >>>>> 1 file changed, 9 insertions(+), 21 deletions(-) > >>>>> > >>>>> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c > >>>>> index 94ba5163d4c5..ed0eecf20de5 100644 > >>>>> --- a/kernel/bpf/verifier.c > >>>>> +++ b/kernel/bpf/verifier.c > >>>>> @@ -7510,6 +7510,15 @@ static int adjust_scalar_min_max_vals(struct bpf_verifier_env *env, > >>>>> u32_min_val = src_reg.u32_min_value; > >>>>> u32_max_val = src_reg.u32_max_value; > >>>>> > >>>>> + if ((opcode == BPF_LSH || opcode == BPF_RSH || opcode == BPF_ARSH) && > >>>>> + umax_val >= insn_bitness) { > >>>>> + /* Shifts greater than 31 or 63 are undefined. > >>>>> + * This includes shifts by a negative number. > >>>>> + */ > >>>>> + verbose(env, "invalid shift %lld\n", umax_val); > >>>>> + return -EINVAL; > >>>>> + } > >>>> > >>>> I think your fix is good. I would like to move after > >>> > >>> I suspect such change will break valid programs that do shift by register. > >>> > >>>> the following code though: > >>>> > >>>> if (!src_known && > >>>> opcode != BPF_ADD && opcode != BPF_SUB && opcode != BPF_AND) { > >>>> __mark_reg_unknown(env, dst_reg); > >>>> return 0; > >>>> } > >>>> > >>>>> + > >>>>> if (alu32) { > >>>>> src_known = tnum_subreg_is_const(src_reg.var_off); > >>>>> if ((src_known && > >>>>> @@ -7592,39 +7601,18 @@ static int adjust_scalar_min_max_vals(struct bpf_verifier_env *env, > >>>>> scalar_min_max_xor(dst_reg, &src_reg); > >>>>> break; > >>>>> case BPF_LSH: > >>>>> - if (umax_val >= insn_bitness) { > >>>>> - /* Shifts greater than 31 or 63 are undefined. > >>>>> - * This includes shifts by a negative number. > >>>>> - */ > >>>>> - mark_reg_unknown(env, regs, insn->dst_reg); > >>>>> - break; > >>>>> - } > >>>> > >>>> I think this is what happens. For the above case, we simply > >>>> marks the dst reg as unknown and didn't fail verification. > >>>> So later on at runtime, the shift optimization will have wrong > >>>> shift value (> 31/64). Please correct me if this is not right > >>>> analysis. As I mentioned in the early please write detailed > >>>> analysis in commit log. > >>> > >>> The large shift is not wrong. It's just undefined. > >>> syzbot has to ignore such cases. > >> > >> Hi Alexei, > >> > >> The report is produced by KUBSAN. I thought there was an agreement on > >> cleaning up KUBSAN reports from the kernel (the subset enabled on > >> syzbot at least). > >> What exactly cases should KUBSAN ignore? > >> +linux-hardening/kasan-dev for KUBSAN false positive > > > > Can check_shl_overflow() be used at all? Best to just make things > > readable and compiler-happy, whatever the implementation. :) > > This is not a compile issue. If the shift amount is a constant, > compiler should have warned and user should fix the warning. > > This is because user code has > something like > a << s; > where s is a unknown variable and > verifier just marked the result of a << s as unknown value. > Verifier may not reject the code depending on how a << s result > is used. > > If bpf program writer uses check_shl_overflow() or some kind > of checking for shift value and won't do shifting if the > shifting may cause an undefined result, there should not > be any kubsan warning. I guess the main question: what should happen if a bpf program writer does _not_ use compiler nor check_shl_overflow()?
On 6/9/21 10:32 PM, Dmitry Vyukov wrote: > On Thu, Jun 10, 2021 at 1:40 AM Yonghong Song <yhs@fb.com> wrote: >> On 6/9/21 11:20 AM, Kees Cook wrote: >>> On Mon, Jun 07, 2021 at 09:38:43AM +0200, 'Dmitry Vyukov' via Clang Built Linux wrote: >>>> On Sat, Jun 5, 2021 at 9:10 PM Alexei Starovoitov >>>> <alexei.starovoitov@gmail.com> wrote: >>>>> On Sat, Jun 5, 2021 at 10:55 AM Yonghong Song <yhs@fb.com> wrote: >>>>>> On 6/5/21 8:01 AM, Kurt Manucredo wrote: >>>>>>> Syzbot detects a shift-out-of-bounds in ___bpf_prog_run() >>>>>>> kernel/bpf/core.c:1414:2. >>>>>> >>>>>> This is not enough. We need more information on why this happens >>>>>> so we can judge whether the patch indeed fixed the issue. >>>>>> >>>>>>> >>>>>>> I propose: In adjust_scalar_min_max_vals() move boundary check up to avoid >>>>>>> missing them and return with error when detected. >>>>>>> >>>>>>> Reported-and-tested-by: syzbot+bed360704c521841c85d@syzkaller.appspotmail.com >>>>>>> Signed-off-by: Kurt Manucredo <fuzzybritches0@gmail.com> >>>>>>> --- >>>>>>> >>>>>>> https://syzkaller.appspot.com/bug?id=edb51be4c9a320186328893287bb30d5eed09231 >>>>>>> >>>>>>> Changelog: >>>>>>> ---------- >>>>>>> v4 - Fix shift-out-of-bounds in adjust_scalar_min_max_vals. >>>>>>> Fix commit message. >>>>>>> v3 - Make it clearer what the fix is for. >>>>>>> v2 - Fix shift-out-of-bounds in ___bpf_prog_run() by adding boundary >>>>>>> check in check_alu_op() in verifier.c. >>>>>>> v1 - Fix shift-out-of-bounds in ___bpf_prog_run() by adding boundary >>>>>>> check in ___bpf_prog_run(). >>>>>>> >>>>>>> thanks >>>>>>> >>>>>>> kind regards >>>>>>> >>>>>>> Kurt >>>>>>> >>>>>>> kernel/bpf/verifier.c | 30 +++++++++--------------------- >>>>>>> 1 file changed, 9 insertions(+), 21 deletions(-) >>>>>>> >>>>>>> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c >>>>>>> index 94ba5163d4c5..ed0eecf20de5 100644 >>>>>>> --- a/kernel/bpf/verifier.c >>>>>>> +++ b/kernel/bpf/verifier.c >>>>>>> @@ -7510,6 +7510,15 @@ static int adjust_scalar_min_max_vals(struct bpf_verifier_env *env, >>>>>>> u32_min_val = src_reg.u32_min_value; >>>>>>> u32_max_val = src_reg.u32_max_value; >>>>>>> >>>>>>> + if ((opcode == BPF_LSH || opcode == BPF_RSH || opcode == BPF_ARSH) && >>>>>>> + umax_val >= insn_bitness) { >>>>>>> + /* Shifts greater than 31 or 63 are undefined. >>>>>>> + * This includes shifts by a negative number. >>>>>>> + */ >>>>>>> + verbose(env, "invalid shift %lld\n", umax_val); >>>>>>> + return -EINVAL; >>>>>>> + } >>>>>> >>>>>> I think your fix is good. I would like to move after >>>>> >>>>> I suspect such change will break valid programs that do shift by register. >>>>> >>>>>> the following code though: >>>>>> >>>>>> if (!src_known && >>>>>> opcode != BPF_ADD && opcode != BPF_SUB && opcode != BPF_AND) { >>>>>> __mark_reg_unknown(env, dst_reg); >>>>>> return 0; >>>>>> } >>>>>> >>>>>>> + >>>>>>> if (alu32) { >>>>>>> src_known = tnum_subreg_is_const(src_reg.var_off); >>>>>>> if ((src_known && >>>>>>> @@ -7592,39 +7601,18 @@ static int adjust_scalar_min_max_vals(struct bpf_verifier_env *env, >>>>>>> scalar_min_max_xor(dst_reg, &src_reg); >>>>>>> break; >>>>>>> case BPF_LSH: >>>>>>> - if (umax_val >= insn_bitness) { >>>>>>> - /* Shifts greater than 31 or 63 are undefined. >>>>>>> - * This includes shifts by a negative number. >>>>>>> - */ >>>>>>> - mark_reg_unknown(env, regs, insn->dst_reg); >>>>>>> - break; >>>>>>> - } >>>>>> >>>>>> I think this is what happens. For the above case, we simply >>>>>> marks the dst reg as unknown and didn't fail verification. >>>>>> So later on at runtime, the shift optimization will have wrong >>>>>> shift value (> 31/64). Please correct me if this is not right >>>>>> analysis. As I mentioned in the early please write detailed >>>>>> analysis in commit log. >>>>> >>>>> The large shift is not wrong. It's just undefined. >>>>> syzbot has to ignore such cases. >>>> >>>> Hi Alexei, >>>> >>>> The report is produced by KUBSAN. I thought there was an agreement on >>>> cleaning up KUBSAN reports from the kernel (the subset enabled on >>>> syzbot at least). >>>> What exactly cases should KUBSAN ignore? >>>> +linux-hardening/kasan-dev for KUBSAN false positive >>> >>> Can check_shl_overflow() be used at all? Best to just make things >>> readable and compiler-happy, whatever the implementation. :) >> >> This is not a compile issue. If the shift amount is a constant, >> compiler should have warned and user should fix the warning. >> >> This is because user code has >> something like >> a << s; >> where s is a unknown variable and >> verifier just marked the result of a << s as unknown value. >> Verifier may not reject the code depending on how a << s result >> is used. >> >> If bpf program writer uses check_shl_overflow() or some kind >> of checking for shift value and won't do shifting if the >> shifting may cause an undefined result, there should not >> be any kubsan warning. > > I guess the main question: what should happen if a bpf program writer > does _not_ use compiler nor check_shl_overflow()? If kubsan is not enabled, everything should work as expected even with shl overflow may cause undefined result. if kubsan is enabled, the reported shift-out-of-bounds warning should be ignored. You could disasm the insn to ensure that there indeed exists a potential shl overflow.
On Wed, Jun 09, 2021 at 11:06:31PM -0700, Yonghong Song wrote: > > > On 6/9/21 10:32 PM, Dmitry Vyukov wrote: > > On Thu, Jun 10, 2021 at 1:40 AM Yonghong Song <yhs@fb.com> wrote: > > > On 6/9/21 11:20 AM, Kees Cook wrote: > > > > On Mon, Jun 07, 2021 at 09:38:43AM +0200, 'Dmitry Vyukov' via Clang Built Linux wrote: > > > > > On Sat, Jun 5, 2021 at 9:10 PM Alexei Starovoitov > > > > > <alexei.starovoitov@gmail.com> wrote: > > > > > > On Sat, Jun 5, 2021 at 10:55 AM Yonghong Song <yhs@fb.com> wrote: > > > > > > > On 6/5/21 8:01 AM, Kurt Manucredo wrote: > > > > > > > > Syzbot detects a shift-out-of-bounds in ___bpf_prog_run() > > > > > > > > kernel/bpf/core.c:1414:2. > > > > > > > [...] > > > > > > > > > > > > > > I think this is what happens. For the above case, we simply > > > > > > > marks the dst reg as unknown and didn't fail verification. > > > > > > > So later on at runtime, the shift optimization will have wrong > > > > > > > shift value (> 31/64). Please correct me if this is not right > > > > > > > analysis. As I mentioned in the early please write detailed > > > > > > > analysis in commit log. > > > > > > > > > > > > The large shift is not wrong. It's just undefined. > > > > > > syzbot has to ignore such cases. > > > > > > > > > > Hi Alexei, > > > > > > > > > > The report is produced by KUBSAN. I thought there was an agreement on > > > > > cleaning up KUBSAN reports from the kernel (the subset enabled on > > > > > syzbot at least). > > > > > What exactly cases should KUBSAN ignore? > > > > > +linux-hardening/kasan-dev for KUBSAN false positive > > > > > > > > Can check_shl_overflow() be used at all? Best to just make things > > > > readable and compiler-happy, whatever the implementation. :) > > > > > > This is not a compile issue. If the shift amount is a constant, > > > compiler should have warned and user should fix the warning. > > > > > > This is because user code has > > > something like > > > a << s; > > > where s is a unknown variable and > > > verifier just marked the result of a << s as unknown value. > > > Verifier may not reject the code depending on how a << s result > > > is used. Ah, gotcha: it's the BPF code itself that needs to catch it. > > > If bpf program writer uses check_shl_overflow() or some kind > > > of checking for shift value and won't do shifting if the > > > shifting may cause an undefined result, there should not > > > be any kubsan warning. Right. > > I guess the main question: what should happen if a bpf program writer > > does _not_ use compiler nor check_shl_overflow()? I think the BPF runtime needs to make such actions defined, instead of doing a blind shift. It needs to check the size of the shift explicitly when handling the shift instruction. > If kubsan is not enabled, everything should work as expected even with > shl overflow may cause undefined result. > > if kubsan is enabled, the reported shift-out-of-bounds warning > should be ignored. You could disasm the insn to ensure that > there indeed exists a potential shl overflow. Sure, but the point of UBSAN is to find and alert about undefined behavior, so we still need to fix this.
On Thu, Jun 10, 2021 at 10:06 AM Kees Cook <keescook@chromium.org> wrote: > > > > I guess the main question: what should happen if a bpf program writer > > > does _not_ use compiler nor check_shl_overflow()? > > I think the BPF runtime needs to make such actions defined, instead of > doing a blind shift. It needs to check the size of the shift explicitly > when handling the shift instruction. Such ideas were brought up in the past and rejected. We're not going to sacrifice performance to make behavior a bit more 'defined'. CPUs are doing it deterministically. It's the C standard that needs fixing. > Sure, but the point of UBSAN is to find and alert about undefined > behavior, so we still need to fix this. No. The undefined behavior of C standard doesn't need "fixing" most of the time.
On Thu, Jun 10, 2021 at 10:52:37AM -0700, Alexei Starovoitov wrote: > On Thu, Jun 10, 2021 at 10:06 AM Kees Cook <keescook@chromium.org> wrote: > > > > > > I guess the main question: what should happen if a bpf program writer > > > > does _not_ use compiler nor check_shl_overflow()? > > > > I think the BPF runtime needs to make such actions defined, instead of > > doing a blind shift. It needs to check the size of the shift explicitly > > when handling the shift instruction. > > Such ideas were brought up in the past and rejected. > We're not going to sacrifice performance to make behavior a bit more > 'defined'. CPUs are doing it deterministically. What CPUs do is not the whole story. The compiler can assume that the shift amount is less than the width and use that assumption in other places, resulting in other things being miscompiled. Couldn't you just AND the shift amounts with the width minus 1? That would make the shifts defined, and the compiler would optimize out the AND on any CPU that interprets the shift amounts modulo the width anyway (e.g., x86). - Eric
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c index 94ba5163d4c5..ed0eecf20de5 100644 --- a/kernel/bpf/verifier.c +++ b/kernel/bpf/verifier.c @@ -7510,6 +7510,15 @@ static int adjust_scalar_min_max_vals(struct bpf_verifier_env *env, u32_min_val = src_reg.u32_min_value; u32_max_val = src_reg.u32_max_value; + if ((opcode == BPF_LSH || opcode == BPF_RSH || opcode == BPF_ARSH) && + umax_val >= insn_bitness) { + /* Shifts greater than 31 or 63 are undefined. + * This includes shifts by a negative number. + */ + verbose(env, "invalid shift %lld\n", umax_val); + return -EINVAL; + } + if (alu32) { src_known = tnum_subreg_is_const(src_reg.var_off); if ((src_known && @@ -7592,39 +7601,18 @@ static int adjust_scalar_min_max_vals(struct bpf_verifier_env *env, scalar_min_max_xor(dst_reg, &src_reg); break; case BPF_LSH: - if (umax_val >= insn_bitness) { - /* Shifts greater than 31 or 63 are undefined. - * This includes shifts by a negative number. - */ - mark_reg_unknown(env, regs, insn->dst_reg); - break; - } if (alu32) scalar32_min_max_lsh(dst_reg, &src_reg); else scalar_min_max_lsh(dst_reg, &src_reg); break; case BPF_RSH: - if (umax_val >= insn_bitness) { - /* Shifts greater than 31 or 63 are undefined. - * This includes shifts by a negative number. - */ - mark_reg_unknown(env, regs, insn->dst_reg); - break; - } if (alu32) scalar32_min_max_rsh(dst_reg, &src_reg); else scalar_min_max_rsh(dst_reg, &src_reg); break; case BPF_ARSH: - if (umax_val >= insn_bitness) { - /* Shifts greater than 31 or 63 are undefined. - * This includes shifts by a negative number. - */ - mark_reg_unknown(env, regs, insn->dst_reg); - break; - } if (alu32) scalar32_min_max_arsh(dst_reg, &src_reg); else
Syzbot detects a shift-out-of-bounds in ___bpf_prog_run() kernel/bpf/core.c:1414:2. I propose: In adjust_scalar_min_max_vals() move boundary check up to avoid missing them and return with error when detected. Reported-and-tested-by: syzbot+bed360704c521841c85d@syzkaller.appspotmail.com Signed-off-by: Kurt Manucredo <fuzzybritches0@gmail.com> --- https://syzkaller.appspot.com/bug?id=edb51be4c9a320186328893287bb30d5eed09231 Changelog: ---------- v4 - Fix shift-out-of-bounds in adjust_scalar_min_max_vals. Fix commit message. v3 - Make it clearer what the fix is for. v2 - Fix shift-out-of-bounds in ___bpf_prog_run() by adding boundary check in check_alu_op() in verifier.c. v1 - Fix shift-out-of-bounds in ___bpf_prog_run() by adding boundary check in ___bpf_prog_run(). thanks kind regards Kurt kernel/bpf/verifier.c | 30 +++++++++--------------------- 1 file changed, 9 insertions(+), 21 deletions(-)