From: John Fastabend <john.fastabend@gmail.com>
To: yhs@fb.com, alexei.starovoitov@gmail.com, daniel@iogearbox.net
Cc: bpf@vger.kernel.org, john.fastabend@gmail.com, kernel-team@fb.com
Subject: [bpf PATCH 1/3] bpf: fix a verifier issue when assigning 32bit reg states to 64bit ones
Date: Fri, 29 May 2020 10:28:40 -0700 [thread overview]
Message-ID: <159077331983.6014.5758956193749002737.stgit@john-Precision-5820-Tower> (raw)
In-Reply-To: <159077324869.6014.6516130782021506562.stgit@john-Precision-5820-Tower>
With the latest trunk llvm (llvm 11), I hit a verifier issue for
test_prog subtest test_verif_scale1.
The following simplified example illustrate the issue:
w9 = 0 /* R9_w=inv0 */
r8 = *(u32 *)(r1 + 80) /* __sk_buff->data_end */
r7 = *(u32 *)(r1 + 76) /* __sk_buff->data */
......
w2 = w9 /* R2_w=inv0 */
r6 = r7 /* R6_w=pkt(id=0,off=0,r=0,imm=0) */
r6 += r2 /* R6_w=inv(id=0) */
r3 = r6 /* R3_w=inv(id=0) */
r3 += 14 /* R3_w=inv(id=0) */
if r3 > r8 goto end
r5 = *(u32 *)(r6 + 0) /* R6_w=inv(id=0) */
<== error here: R6 invalid mem access 'inv'
...
end:
In real test_verif_scale1 code, "w9 = 0" and "w2 = w9" are in
different basic blocks.
In the above, after "r6 += r2", r6 becomes a scalar, which eventually
caused the memory access error. The correct register state should be
a pkt pointer.
The inprecise register state starts at "w2 = w9".
The 32bit register w9 is 0, in __reg_assign_32_into_64(),
the 64bit reg->smax_value is assigned to be U32_MAX.
The 64bit reg->smin_value is 0 and the 64bit register
itself remains constant based on reg->var_off.
In adjust_ptr_min_max_vals(), the verifier checks for a known constant,
smin_val must be equal to smax_val. Since they are not equal,
the verifier decides r6 is a unknown scalar, which caused later failure.
The llvm10 does not have this issue as it generates different code:
w9 = 0 /* R9_w=inv0 */
r8 = *(u32 *)(r1 + 80) /* __sk_buff->data_end */
r7 = *(u32 *)(r1 + 76) /* __sk_buff->data */
......
r6 = r7 /* R6_w=pkt(id=0,off=0,r=0,imm=0) */
r6 += r9 /* R6_w=pkt(id=0,off=0,r=0,imm=0) */
r3 = r6 /* R3_w=pkt(id=0,off=0,r=0,imm=0) */
r3 += 14 /* R3_w=pkt(id=0,off=14,r=0,imm=0) */
if r3 > r8 goto end
...
To fix the above issue, we can include zero in the test condition for
assigning the s32_max_value and s32_min_value to their 64-bit equivalents
smax_value and smin_value.
Further, fix the condition to avoid doing zero extension bounds checks
when s32_min_value <= 0. This could allow for the case where bounds
32-bit bounds (-1,1) get incorrectly translated to (0,1) 64-bit bounds.
When in-fact the -1 min value needs to force U32_MAX bound.
Fixes: 3f50f132d840 ("bpf: Verifier, do explicit ALU32 bounds tracking")
Signed-off-by: John Fastabend <john.fastabend@gmail.com>
---
kernel/bpf/verifier.c | 10 +++++-----
1 file changed, 5 insertions(+), 5 deletions(-)
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index d2e27db..d0bdd55 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -1217,14 +1217,14 @@ static void __reg_assign_32_into_64(struct bpf_reg_state *reg)
* but must be positive otherwise set to worse case bounds
* and refine later from tnum.
*/
- if (reg->s32_min_value > 0)
- reg->smin_value = reg->s32_min_value;
- else
- reg->smin_value = 0;
- if (reg->s32_max_value > 0)
+ if (reg->s32_min_value >= 0 && reg->s32_max_value >= 0)
reg->smax_value = reg->s32_max_value;
else
reg->smax_value = U32_MAX;
+ if (reg->s32_min_value >= 0)
+ reg->smin_value = reg->s32_min_value;
+ else
+ reg->smin_value = 0;
}
static void __reg_combine_32_into_64(struct bpf_reg_state *reg)
next prev parent reply other threads:[~2020-05-29 17:37 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-05-29 17:28 [bpf PATCH 0/3] verifier fix for assigning 32bit reg to 64bit reg John Fastabend
2020-05-29 17:28 ` John Fastabend [this message]
2020-05-29 18:40 ` [bpf PATCH 1/3] bpf: fix a verifier issue when assigning 32bit reg states to 64bit ones Yonghong Song
2020-05-29 17:28 ` [bpf PATCH 2/3] bpf, selftests: verifier bounds tests need to be updated John Fastabend
2020-05-29 18:41 ` Yonghong Song
2020-05-29 17:29 ` [bpf PATCH 3/3] bpf, selftests: add a verifier test for assigning 32bit reg states to 64bit ones John Fastabend
2020-05-29 20:53 ` [bpf PATCH 0/3] verifier fix for assigning 32bit reg to 64bit reg Alexei Starovoitov
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