This is a note to let you know that I've just added the patch titled
bpf: Fix verification of indirect var-off stack access
to the 6.1-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
bpf-fix-verification-of-indirect-var-off-stack-acces.patch
and it can be found in the queue-6.1 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
commit 7b4ec453d802ce53d96da6be739a2a011d72b3c3
Author: Andrei Matei <andreimatei1@xxxxxxxxx>
Date: Wed Dec 6 23:11:48 2023 -0500
bpf: Fix verification of indirect var-off stack access
[ Upstream commit a833a17aeac73b33f79433d7cee68d5cafd71e4f ]
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!
Fixes: 01f810ace9ed3 ("bpf: Allow variable-offset stack access")
Reported-by: Hao Sun <sunhao.th@xxxxxxxxx>
Signed-off-by: Andrei Matei <andreimatei1@xxxxxxxxx>
Signed-off-by: Andrii Nakryiko <andrii@xxxxxxxxxx>
Acked-by: Eduard Zingerman <eddyz87@xxxxxxxxx>
Acked-by: Andrii Nakryiko <andrii@xxxxxxxxxx>
Link: https://lore.kernel.org/bpf/20231207041150.229139-2-andreimatei1@xxxxxxxxx
Closes: https://lore.kernel.org/bpf/CACkBjsZGEUaRCHsmaX=h-efVogsRfK1FPxmkgb0Os_frnHiNdw@xxxxxxxxxxxxxx/
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index adadf8546270..95e7b638418e 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -4950,10 +4950,7 @@ 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) {
@@ -4962,15 +4959,12 @@ 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)) {
