diff mbox series

[bpf-next,v5,1/3] bpf: fix verification of indirect var-off stack access

Message ID 20231207041150.229139-2-andreimatei1@gmail.com (mailing list archive)
State Accepted
Commit 4f114bc280bb327590ba964985257e9150ae6956
Delegated to: BPF
Headers show
Series bpf: fix verification of indirect var-off stack access | expand

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netdev/ynl success SINGLE THREAD; Generated files up to date; no warnings/errors; no diff in generated;
netdev/fixes_present success Fixes tag not required for -next series
netdev/header_inline success No static functions without inline keyword in header files
netdev/build_32bit success Errors and warnings before: 1127 this patch: 1127
netdev/cc_maintainers fail 1 blamed authors not CCed: ast@kernel.org; 10 maintainers not CCed: kpsingh@kernel.org daniel@iogearbox.net yonghong.song@linux.dev sdf@google.com martin.lau@linux.dev song@kernel.org ast@kernel.org haoluo@google.com jolsa@kernel.org john.fastabend@gmail.com
netdev/build_clang success Errors and warnings before: 1147 this patch: 1147
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netdev/deprecated_api success None detected
netdev/check_selftest success No net selftest shell script
netdev/verify_fixes success Fixes tag looks correct
netdev/build_allmodconfig_warn success Errors and warnings before: 1154 this patch: 1154
netdev/checkpatch warning WARNING: Please use correct Fixes: style 'Fixes: <12 chars of sha1> ("<title line>")' - ie: 'Fixes: 01f810ace9ed ("bpf: Allow variable-offset stack access")' WARNING: line length of 82 exceeds 80 columns
netdev/build_clang_rust success No Rust files in patch. Skipping build
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Commit Message

Andrei Matei Dec. 7, 2023, 4:11 a.m. UTC 
This patch fixes a bug around the verification of possibly-zero-sized
stack accesses. When the access was done through a var-offset stack
pointer, check_stack_access_within_bounds was incorrectly computing the
maximum-offset of a zero-sized read to be the same as the register's min
offset. Instead, we have to take in account the register's maximum
possible value. The patch also simplifies how the max offset is checked;
the check is now simpler than for min offset.

The bug was allowing accesses to erroneously pass the
check_stack_access_within_bounds() checks, only to later crash in
check_stack_range_initialized() when all the possibly-affected stack
slots are iterated (this time with a correct max offset).
check_stack_range_initialized() is relying on
check_stack_access_within_bounds() for its accesses to the
stack-tracking vector to be within bounds; in the case of zero-sized
accesses, we were essentially only verifying that the lowest possible
slot was within bounds. We would crash when the max-offset of the stack
pointer was >= 0 (which shouldn't pass verification, and hopefully is
not something anyone's code attempts to do in practice).

Thanks Hao for reporting!

Reported-by: Hao Sun <sunhao.th@gmail.com>
Fixes: 01f810ace9ed3 ("bpf: Allow variable-offset stack access")
Closes: https://lore.kernel.org/bpf/CACkBjsZGEUaRCHsmaX=h-efVogsRfK1FPxmkgb0Os_frnHiNdw@mail.gmail.com/
Signed-off-by: Andrei Matei <andreimatei1@gmail.com>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
---
 kernel/bpf/verifier.c | 14 ++++----------
 1 file changed, 4 insertions(+), 10 deletions(-)
diff mbox series

Patch

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index e5ce530641ba..137240681fa9 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -6620,10 +6620,7 @@  static int check_stack_access_within_bounds(
 
 	if (tnum_is_const(reg->var_off)) {
 		min_off = reg->var_off.value + off;
-		if (access_size > 0)
-			max_off = min_off + access_size - 1;
-		else
-			max_off = min_off;
+		max_off = min_off + access_size;
 	} else {
 		if (reg->smax_value >= BPF_MAX_VAR_OFF ||
 		    reg->smin_value <= -BPF_MAX_VAR_OFF) {
@@ -6632,15 +6629,12 @@  static int check_stack_access_within_bounds(
 			return -EACCES;
 		}
 		min_off = reg->smin_value + off;
-		if (access_size > 0)
-			max_off = reg->smax_value + off + access_size - 1;
-		else
-			max_off = min_off;
+		max_off = reg->smax_value + off + access_size;
 	}
 
 	err = check_stack_slot_within_bounds(min_off, state, type);
-	if (!err)
-		err = check_stack_slot_within_bounds(max_off, state, type);
+	if (!err && max_off > 0)
+		err = -EINVAL; /* out of stack access into non-negative offsets */
 
 	if (err) {
 		if (tnum_is_const(reg->var_off)) {