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: 3f50f132d8 ("bpf: Verifier, do explicit ALU32 bounds tracking")
Signed-off-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/159077331983.6014.5758956193749002737.stgit@john-Precision-5820-Tower
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