Yi Lai reported an issue ([1]) where the following warning appears
in kernel dmesg:
[ 60.643604] verifier backtracking bug
[ 60.643635] WARNING: CPU: 10 PID: 2315 at kernel/bpf/verifier.c:4302 __mark_chain_precision+0x3a6c/0x3e10
[ 60.648428] Modules linked in: bpf_testmod(OE)
[ 60.650471] CPU: 10 UID: 0 PID: 2315 Comm: test_progs Tainted: G OE 6.15.0-rc4-gef11287f8289-dirty #327 PREEMPT(full)
[ 60.654385] Tainted: [O]=OOT_MODULE, [E]=UNSIGNED_MODULE
[ 60.656682] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.14.0-0-g155821a1990b-prebuilt.qemu.org 04/01/2014
[ 60.660475] RIP: 0010:__mark_chain_precision+0x3a6c/0x3e10
[ 60.662814] Code: 5a 30 84 89 ea e8 c4 d9 01 00 80 3d 3e 7d d8 04 00 0f 85 60 fa ff ff c6 05 31 7d d8 04
01 48 c7 c7 00 58 30 84 e8 c4 06 a5 ff <0f> 0b e9 46 fa ff ff 48 ...
[ 60.668720] RSP: 0018:ffff888116cc7298 EFLAGS: 00010246
[ 60.671075] RAX: 54d70e82dfd31900 RBX: ffff888115b65e20 RCX: 0000000000000000
[ 60.673659] RDX: 0000000000000001 RSI: 0000000000000004 RDI: 00000000ffffffff
[ 60.676241] RBP: 0000000000000400 R08: ffff8881f6f23bd3 R09: 1ffff1103ede477a
[ 60.678787] R10: dffffc0000000000 R11: ffffed103ede477b R12: ffff888115b60ae8
[ 60.681420] R13: 1ffff11022b6cbc4 R14: 00000000fffffff2 R15: 0000000000000001
[ 60.684030] FS: 00007fc2aedd80c0(0000) GS:ffff88826fa8a000(0000) knlGS:0000000000000000
[ 60.686837] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 60.689027] CR2: 000056325369e000 CR3: 000000011088b002 CR4: 0000000000370ef0
[ 60.691623] Call Trace:
[ 60.692821] <TASK>
[ 60.693960] ? __pfx_verbose+0x10/0x10
[ 60.695656] ? __pfx_disasm_kfunc_name+0x10/0x10
[ 60.697495] check_cond_jmp_op+0x16f7/0x39b0
[ 60.699237] do_check+0x58fa/0xab10
...
Further analysis shows the warning is at line 4302 as below:
4294 /* static subprog call instruction, which
4295 * means that we are exiting current subprog,
4296 * so only r1-r5 could be still requested as
4297 * precise, r0 and r6-r10 or any stack slot in
4298 * the current frame should be zero by now
4299 */
4300 if (bt_reg_mask(bt) & ~BPF_REGMASK_ARGS) {
4301 verbose(env, "BUG regs %x\n", bt_reg_mask(bt));
4302 WARN_ONCE(1, "verifier backtracking bug");
4303 return -EFAULT;
4304 }
With the below test (also in the next patch):
__used __naked static void __bpf_jmp_r10(void)
{
asm volatile (
"r2 = 2314885393468386424 ll;"
"goto +0;"
"if r2 <= r10 goto +3;"
"if r1 >= -1835016 goto +0;"
"if r2 <= 8 goto +0;"
"if r3 <= 0 goto +0;"
"exit;"
::: __clobber_all);
}
SEC("?raw_tp")
__naked void bpf_jmp_r10(void)
{
asm volatile (
"r3 = 0 ll;"
"call __bpf_jmp_r10;"
"r0 = 0;"
"exit;"
::: __clobber_all);
}
The following is the verifier failure log:
0: (18) r3 = 0x0 ; R3_w=0
2: (85) call pc+2
caller:
R10=fp0
callee:
frame1: R1=ctx() R3_w=0 R10=fp0
5: frame1: R1=ctx() R3_w=0 R10=fp0
; asm volatile (" \ @ verifier_precision.c:184
5: (18) r2 = 0x20202000256c6c78 ; frame1: R2_w=0x20202000256c6c78
7: (05) goto pc+0
8: (bd) if r2 <= r10 goto pc+3 ; frame1: R2_w=0x20202000256c6c78 R10=fp0
9: (35) if r1 >= 0xffe3fff8 goto pc+0 ; frame1: R1=ctx()
10: (b5) if r2 <= 0x8 goto pc+0
mark_precise: frame1: last_idx 10 first_idx 0 subseq_idx -1
mark_precise: frame1: regs=r2 stack= before 9: (35) if r1 >= 0xffe3fff8 goto pc+0
mark_precise: frame1: regs=r2 stack= before 8: (bd) if r2 <= r10 goto pc+3
mark_precise: frame1: regs=r2,r10 stack= before 7: (05) goto pc+0
mark_precise: frame1: regs=r2,r10 stack= before 5: (18) r2 = 0x20202000256c6c78
mark_precise: frame1: regs=r10 stack= before 2: (85) call pc+2
BUG regs 400
The main failure reason is due to r10 in precision backtracking bookkeeping.
Actually r10 is always precise and there is no need to add it for the precision
backtracking bookkeeping.
One way to fix the issue is to prevent bt_set_reg() if any src/dst reg is
r10. Andrii suggested to go with push_insn_history() approach to avoid
explicitly checking r10 in backtrack_insn().
This patch added push_insn_history() support for cond_jmp like 'rX <op> rY'
operations. In check_cond_jmp_op(), if any of rX or rY is a stack pointer,
push_insn_history() will record such information, and later backtrack_insn()
will do bt_set_reg() properly for those register(s).
[1] https://lore.kernel.org/bpf/Z%2F8q3xzpU59CIYQE@ly-workstation/
Reported by: Yi Lai <yi1.lai@xxxxxxxxxxxxxxx>
Fixes: 407958a0e980 ("bpf: encapsulate precision backtracking bookkeeping")
Signed-off-by: Yonghong Song <yonghong.song@xxxxxxxxx>
---
include/linux/bpf_verifier.h | 12 ++++++++----
kernel/bpf/verifier.c | 18 ++++++++++++++++--
2 files changed, 24 insertions(+), 6 deletions(-)
Changelogs:
v4 -> v5:
- v4: https://lore.kernel.org/bpf/20250522050239.2834718-1-yonghong.song@xxxxxxxxx/
- Simplify implementation in backtrack_insn() and check_cond_jmp_op().
v3 -> v4:
- v3: https://lore.kernel.org/bpf/20250521170409.2772304-1-yonghong.song@xxxxxxxxx/
- Fix an issue in push_cond_jmp_history(). Previously, '!src_reg' was used to
check whether insn is 'dreg <op> imm' or not. But actually '!src_reg' is always
non-NULL. The new fix is using insn directly.
v2 -> v3:
- v2: https://lore.kernel.org/bpf/20250516161029.962760-1-yonghong.song@xxxxxxxxx/
- In v2, I put sreg_flag/dreg_flag into bpf_insn_hist_entry and the information
includes register numbers. This is not necessary as later insn in backtracking
can retrieve the register number. So the new change is remove sreg_flag/dreg_flag
from bpf_insn_hist_entry and add two bits in bpf_insn_hist_entry.flags to
record whether the registers (cond jump like <reg> op < reg>) are stack pointer
or not. Other changes depend on this data structure change.
v1 -> v2:
- v1: https://lore.kernel.org/bpf/20250511162758.281071-1-yonghong.song@xxxxxxxxx/
- In v1, we check r10 register explicitly in backtrack_insn() to decide
whether we should do bt_set_reg() or not. Andrii suggested to do
push_insn_history() instead. Whether a particular register (r10 in this case)
should be available for backtracking or not is in check_cond_jmp_op(),
and such information is pushed with push_insn_history(). Later in backtrack_insn(),
such info is retrieved to decide whether precision marking should be
done or not. This apporach can avoid explicit checking for r10 in backtrack_insn().
diff --git a/include/linux/bpf_verifier.h b/include/linux/bpf_verifier.h
index 78c97e12ea4e..256274acb1d8 100644
--- a/include/linux/bpf_verifier.h
+++ b/include/linux/bpf_verifier.h
@@ -356,7 +356,11 @@ enum {
INSN_F_SPI_MASK = 0x3f, /* 6 bits */
INSN_F_SPI_SHIFT = 3, /* shifted 3 bits to the left */
- INSN_F_STACK_ACCESS = BIT(9), /* we need 10 bits total */
+ INSN_F_STACK_ACCESS = BIT(9),
+
+ INSN_F_DST_REG_STACK = BIT(10), /* dst_reg is PTR_TO_STACK */
+ INSN_F_SRC_REG_STACK = BIT(11), /* src_reg is PTR_TO_STACK */
+ /* total 12 bits are used now. */
};
static_assert(INSN_F_FRAMENO_MASK + 1 >= MAX_CALL_FRAMES);
@@ -365,9 +369,9 @@ static_assert(INSN_F_SPI_MASK + 1 >= MAX_BPF_STACK / 8);
struct bpf_insn_hist_entry {
u32 idx;
/* insn idx can't be bigger than 1 million */
- u32 prev_idx : 22;
- /* special flags, e.g., whether insn is doing register stack spill/load */
- u32 flags : 10;
+ u32 prev_idx : 20;
+ /* special INSN_F_xxx flags */
+ u32 flags : 12;
/* additional registers that need precision tracking when this
* jump is backtracked, vector of six 10-bit records
*/
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index d5807d2efc92..831c2eff56e1 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -4410,8 +4410,10 @@ static int backtrack_insn(struct bpf_verifier_env *env, int idx, int subseq_idx,
* before it would be equally necessary to
* propagate it to dreg.
*/
- bt_set_reg(bt, dreg);
- bt_set_reg(bt, sreg);
+ if (!hist || !(hist->flags & INSN_F_SRC_REG_STACK))
+ bt_set_reg(bt, sreg);
+ if (!hist || !(hist->flags & INSN_F_DST_REG_STACK))
+ bt_set_reg(bt, dreg);
} else if (BPF_SRC(insn->code) == BPF_K) {
/* dreg <cond> K
* Only dreg still needs precision before
@@ -16407,6 +16409,7 @@ static int check_cond_jmp_op(struct bpf_verifier_env *env,
struct bpf_reg_state *eq_branch_regs;
struct linked_regs linked_regs = {};
u8 opcode = BPF_OP(insn->code);
+ int insn_flags = 0;
bool is_jmp32;
int pred = -1;
int err;
@@ -16465,6 +16468,9 @@ static int check_cond_jmp_op(struct bpf_verifier_env *env,
insn->src_reg);
return -EACCES;
}
+
+ if (src_reg->type == PTR_TO_STACK)
+ insn_flags |= INSN_F_SRC_REG_STACK;
} else {
if (insn->src_reg != BPF_REG_0) {
verbose(env, "BPF_JMP/JMP32 uses reserved fields\n");
@@ -16476,6 +16482,14 @@ static int check_cond_jmp_op(struct bpf_verifier_env *env,
__mark_reg_known(src_reg, insn->imm);
}
+ if (dst_reg->type == PTR_TO_STACK)
+ insn_flags |= INSN_F_DST_REG_STACK;
+ if (insn_flags) {
+ err = push_insn_history(env, this_branch, insn_flags, 0);
+ if (err)
+ return err;
+ }
+
is_jmp32 = BPF_CLASS(insn->code) == BPF_JMP32;
pred = is_branch_taken(dst_reg, src_reg, opcode, is_jmp32);
if (pred >= 0) {
--
2.47.1
