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bpf: Fix packet range of pointers sharing an id

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Since commit 022ac07508 ("bpf: use reg->var_off instead of reg->off
for pointers"), find_good_pkt_pointers() sets the range of all packet
pointers sharing an id from the umax of the compared pointer, and
check_packet_access() requires umax + off + size <= range.  That assumes
the umax of two such pointers differ by exactly their constant distance.
reg_bounds_sync() breaks it when var_off tightens one umax and not the
other:

	r4 &= 0x38
	if r4 > 50 goto exit     ; umax 50, var_off (0x0; 0x38)
	r5 = pkt + r4            ; umax 50
	r6 = r5
	r6 += 8                  ; umax 56, not 58

Comparing r6 with pkt_end sets the range to 56, and the valid 8-byte
load at r5 is rejected (50 + 8 > 56).  Comparing r5 sets it to 50, and
the out-of-bounds 1-byte load at r6 - 7, i.e. r5 + 1, is accepted
(56 - 7 + 1 <= 50).

Don't call reg_bounds_sync() on a packet pointer that keeps its id (a
constant was added or subtracted) or its range (an unknown non-negative
value was subtracted), so that var_off cannot tighten its umax.  Only
update the 32-bit bounds from var_off: reg_bounds_sanity_check() wants
them constant when the lower half of var_off is, e.g. for pkt + 8.

This relies on nothing else changing the 64-bit bounds of a packet
pointer, which holds today.

var_off of such a pointer is no longer narrowed by its bounds.  Adjust
three verifier_align expectations; the low bits, which the alignment
checks use, don't change.  veristat on the selftests shows no verdict
changes and +0.8% insns in test_cls_redirect_subprogs.

Fixes: 022ac07508 ("bpf: use reg->var_off instead of reg->off for pointers")
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20261001145255.855630-1-alexei.starovoitov@gmail.com
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
This commit is contained in:
Alexei Starovoitov 2026-10-01 14:52:54 +00:00 committed by Kumar Kartikeya Dwivedi
parent 564b7e8e09
commit b88822d258
No known key found for this signature in database
GPG Key ID: 472D377B63542F83
2 changed files with 12 additions and 4 deletions
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10
kernel/bpf/verifier.c
View File

@ -14826,7 +14826,15 @@ static int adjust_ptr_min_max_vals(struct bpf_verifier_env *env, struct bpf_insn
"Tighten the scalar bounds before the arithmetic so the resulting pointer remains within the allowed range.");
return -EINVAL;
}
reg_bounds_sync(dst_reg);
/*
* A packet pointer that keeps its id or range is checked against a
* range set from the checked pointer's umax, so var_off must not tighten
* its umax. r32 must still match var_off for reg_bounds_sanity_check().
*/
if (reg_is_pkt_pointer(dst_reg) && (known || dst_reg->range > 0))
__update_reg32_bounds(dst_reg);
else
reg_bounds_sync(dst_reg);
bounds_ret = sanitize_check_bounds(env, insn, dst_reg);
if (bounds_ret == -EACCES)
return bounds_ret;

6
tools/testing/selftests/bpf/progs/verifier_align.c
View File

@ -284,7 +284,7 @@ __msg("26: {{.*}} R5=pkt(r=8,imm=14)")
*/
__msg("28: {{.*}} R4={{[^)]*}}var_off=(0x2; 0x7fc){{.*}} R5={{[^)]*}}var_off=(0x2; 0x7fc)")
/* Constant is added to R5 again, setting reg->off to 18. */
__msg("29: {{.*}} R5=pkt(id=3,{{[^)]*}}var_off=(0x2; 0x7fc)")
__msg("29: {{.*}} R5=pkt(id=3,{{[^)]*}}var_off=(0x2; 0xffc)")
/* And once more we add a variable; resulting {{[^)]*}}var_off
* is still (4n), fixed offset is not changed.
* Also, we create a new reg->id.
@ -359,7 +359,7 @@ __msg("7: {{.*}} R6={{[^)]*}}var_off=(0x0; 0x3fc)")
__msg("8: {{.*}} R6={{[^)]*}}var_off=(0x2; 0x7fc)")
/* Packet pointer has (4n+2) offset */
__msg("11: {{.*}} R5={{[^)]*}}var_off=(0x2; 0x7fc)")
__msg("12: {{.*}} R4={{[^)]*}}var_off=(0x2; 0x7fc)")
__msg("12: {{.*}} R4={{[^)]*}}var_off=(0x2; 0xffc)")
/* At the time the word size load is performed from R5,
* its total fixed offset is NET_IP_ALIGN + reg->off (0)
* which is 2. Then the variable offset is (4n+2), so
@ -375,7 +375,7 @@ __msg("17: {{.*}} R6={{[^)]*}}var_off=(0x0; 0x3fc)")
* another (4n+2).
*/
__msg("19: {{.*}} R5={{[^)]*}}var_off=(0x2; 0xffc)")
__msg("20: {{.*}} R4={{[^)]*}}var_off=(0x2; 0xffc)")
__msg("20: {{.*}} R4={{[^)]*}}var_off=(0x2; 0x1ffc)")
/* At the time the word size load is performed from R5,
* its total fixed offset is NET_IP_ALIGN + reg->off (0)
* which is 2. Then the variable offset is (4n+2), so