| Message ID | 20230706233248.445713-1-rick.p.edgecombe@intel.com (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | x86/shstk: Move arch detail comment out of core mm | expand |
On Thu, Jul 06, 2023 at 04:32:48PM -0700, Rick Edgecombe wrote: > The comment around VM_SHADOW_STACK in mm.h refers to a lot of x86 > specific details that don't belong in a cross arch file. Remove these > out of core mm, and just leave the non-arch details. > > Since the comment includes some useful details that would be good to > retain in the source somewhere, put the arch specifics parts in > arch/x86/shstk.c near alloc_shstk(), where memory of this type is > allocated. Include a reference to the existence of the x86 details near > the VM_SHADOW_STACK definition mm.h. Reviewed-by: Mark Brown <broonie@kernel.org>
Hi Dave, Rick, It seems it didn't get into the current tip. On Thu, Jul 06, 2023 at 04:32:48PM -0700, Rick Edgecombe wrote: > The comment around VM_SHADOW_STACK in mm.h refers to a lot of x86 > specific details that don't belong in a cross arch file. Remove these > out of core mm, and just leave the non-arch details. > > Since the comment includes some useful details that would be good to > retain in the source somewhere, put the arch specifics parts in > arch/x86/shstk.c near alloc_shstk(), where memory of this type is > allocated. Include a reference to the existence of the x86 details near > the VM_SHADOW_STACK definition mm.h. > > Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com> > --- > arch/x86/kernel/shstk.c | 25 +++++++++++++++++++++++++ > include/linux/mm.h | 32 ++++++-------------------------- > 2 files changed, 31 insertions(+), 26 deletions(-) > > diff --git a/arch/x86/kernel/shstk.c b/arch/x86/kernel/shstk.c > index b26810c7cd1c..47f5204b0fa9 100644 > --- a/arch/x86/kernel/shstk.c > +++ b/arch/x86/kernel/shstk.c > @@ -72,6 +72,31 @@ static int create_rstor_token(unsigned long ssp, unsigned long *token_addr) > return 0; > } > > +/* > + * VM_SHADOW_STACK will have a guard page. This helps userspace protect > + * itself from attacks. The reasoning is as follows: > + * > + * The shadow stack pointer(SSP) is moved by CALL, RET, and INCSSPQ. The > + * INCSSP instruction can increment the shadow stack pointer. It is the > + * shadow stack analog of an instruction like: > + * > + * addq $0x80, %rsp > + * > + * However, there is one important difference between an ADD on %rsp > + * and INCSSP. In addition to modifying SSP, INCSSP also reads from the > + * memory of the first and last elements that were "popped". It can be > + * thought of as acting like this: > + * > + * READ_ONCE(ssp); // read+discard top element on stack > + * ssp += nr_to_pop * 8; // move the shadow stack > + * READ_ONCE(ssp-8); // read+discard last popped stack element > + * > + * The maximum distance INCSSP can move the SSP is 2040 bytes, before > + * it would read the memory. Therefore a single page gap will be enough > + * to prevent any operation from shifting the SSP to an adjacent stack, > + * since it would have to land in the gap at least once, causing a > + * fault. > + */ > static unsigned long alloc_shstk(unsigned long addr, unsigned long size, > unsigned long token_offset, bool set_res_tok) > { > diff --git a/include/linux/mm.h b/include/linux/mm.h > index 535c58d3b2e4..b647cf2e94ea 100644 > --- a/include/linux/mm.h > +++ b/include/linux/mm.h > @@ -343,33 +343,13 @@ extern unsigned int kobjsize(const void *objp); > > #ifdef CONFIG_X86_USER_SHADOW_STACK > /* > - * This flag should not be set with VM_SHARED because of lack of support > - * core mm. It will also get a guard page. This helps userspace protect > - * itself from attacks. The reasoning is as follows: > + * VM_SHADOW_STACK should not be set with VM_SHARED because of lack of > + * support core mm. > * > - * The shadow stack pointer(SSP) is moved by CALL, RET, and INCSSPQ. The > - * INCSSP instruction can increment the shadow stack pointer. It is the > - * shadow stack analog of an instruction like: > - * > - * addq $0x80, %rsp > - * > - * However, there is one important difference between an ADD on %rsp > - * and INCSSP. In addition to modifying SSP, INCSSP also reads from the > - * memory of the first and last elements that were "popped". It can be > - * thought of as acting like this: > - * > - * READ_ONCE(ssp); // read+discard top element on stack > - * ssp += nr_to_pop * 8; // move the shadow stack > - * READ_ONCE(ssp-8); // read+discard last popped stack element > - * > - * The maximum distance INCSSP can move the SSP is 2040 bytes, before > - * it would read the memory. Therefore a single page gap will be enough > - * to prevent any operation from shifting the SSP to an adjacent stack, > - * since it would have to land in the gap at least once, causing a > - * fault. > - * > - * Prevent using INCSSP to move the SSP between shadow stacks by > - * having a PAGE_SIZE guard gap. > + * These VMAs will get a single end guard page. This helps userspace protect > + * itself from attacks. A single page is enough for current shadow stack archs > + * (x86). See the comments near alloc_shstk() in arch/x86/kernel/shstk.c > + * for more details on the guard size. > */ > # define VM_SHADOW_STACK VM_HIGH_ARCH_5 > #else > -- > 2.34.1 >
diff --git a/arch/x86/kernel/shstk.c b/arch/x86/kernel/shstk.c index b26810c7cd1c..47f5204b0fa9 100644 --- a/arch/x86/kernel/shstk.c +++ b/arch/x86/kernel/shstk.c @@ -72,6 +72,31 @@ static int create_rstor_token(unsigned long ssp, unsigned long *token_addr) return 0; } +/* + * VM_SHADOW_STACK will have a guard page. This helps userspace protect + * itself from attacks. The reasoning is as follows: + * + * The shadow stack pointer(SSP) is moved by CALL, RET, and INCSSPQ. The + * INCSSP instruction can increment the shadow stack pointer. It is the + * shadow stack analog of an instruction like: + * + * addq $0x80, %rsp + * + * However, there is one important difference between an ADD on %rsp + * and INCSSP. In addition to modifying SSP, INCSSP also reads from the + * memory of the first and last elements that were "popped". It can be + * thought of as acting like this: + * + * READ_ONCE(ssp); // read+discard top element on stack + * ssp += nr_to_pop * 8; // move the shadow stack + * READ_ONCE(ssp-8); // read+discard last popped stack element + * + * The maximum distance INCSSP can move the SSP is 2040 bytes, before + * it would read the memory. Therefore a single page gap will be enough + * to prevent any operation from shifting the SSP to an adjacent stack, + * since it would have to land in the gap at least once, causing a + * fault. + */ static unsigned long alloc_shstk(unsigned long addr, unsigned long size, unsigned long token_offset, bool set_res_tok) { diff --git a/include/linux/mm.h b/include/linux/mm.h index 535c58d3b2e4..b647cf2e94ea 100644 --- a/include/linux/mm.h +++ b/include/linux/mm.h @@ -343,33 +343,13 @@ extern unsigned int kobjsize(const void *objp); #ifdef CONFIG_X86_USER_SHADOW_STACK /* - * This flag should not be set with VM_SHARED because of lack of support - * core mm. It will also get a guard page. This helps userspace protect - * itself from attacks. The reasoning is as follows: + * VM_SHADOW_STACK should not be set with VM_SHARED because of lack of + * support core mm. * - * The shadow stack pointer(SSP) is moved by CALL, RET, and INCSSPQ. The - * INCSSP instruction can increment the shadow stack pointer. It is the - * shadow stack analog of an instruction like: - * - * addq $0x80, %rsp - * - * However, there is one important difference between an ADD on %rsp - * and INCSSP. In addition to modifying SSP, INCSSP also reads from the - * memory of the first and last elements that were "popped". It can be - * thought of as acting like this: - * - * READ_ONCE(ssp); // read+discard top element on stack - * ssp += nr_to_pop * 8; // move the shadow stack - * READ_ONCE(ssp-8); // read+discard last popped stack element - * - * The maximum distance INCSSP can move the SSP is 2040 bytes, before - * it would read the memory. Therefore a single page gap will be enough - * to prevent any operation from shifting the SSP to an adjacent stack, - * since it would have to land in the gap at least once, causing a - * fault. - * - * Prevent using INCSSP to move the SSP between shadow stacks by - * having a PAGE_SIZE guard gap. + * These VMAs will get a single end guard page. This helps userspace protect + * itself from attacks. A single page is enough for current shadow stack archs + * (x86). See the comments near alloc_shstk() in arch/x86/kernel/shstk.c + * for more details on the guard size. */ # define VM_SHADOW_STACK VM_HIGH_ARCH_5 #else
The comment around VM_SHADOW_STACK in mm.h refers to a lot of x86 specific details that don't belong in a cross arch file. Remove these out of core mm, and just leave the non-arch details. Since the comment includes some useful details that would be good to retain in the source somewhere, put the arch specifics parts in arch/x86/shstk.c near alloc_shstk(), where memory of this type is allocated. Include a reference to the existence of the x86 details near the VM_SHADOW_STACK definition mm.h. Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com> --- arch/x86/kernel/shstk.c | 25 +++++++++++++++++++++++++ include/linux/mm.h | 32 ++++++-------------------------- 2 files changed, 31 insertions(+), 26 deletions(-)