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[PATCH bpf-next v2 1/2] bpf: Skip bounds adjustment for conditional jumps on same register

KaFai Wan posted 2 patches 3 months, 2 weeks ago
[PATCH bpf-next v2 1/2] bpf: Skip bounds adjustment for conditional jumps on same register
Posted by KaFai Wan 3 months, 2 weeks ago 
When conditional jumps are performed on the same register (e.g., r0 <= r0,
r0 > r0, r0 < r0) where the register holds a scalar with range, the verifier
incorrectly attempts to adjust the register's min/max bounds. This leads to
invalid range bounds and triggers a BUG warning:

verifier bug: REG INVARIANTS VIOLATION (true_reg1): range bounds violation u64=[0x1, 0x0] s64=[0x1, 0x0] u32=[0x1, 0x0] s32=[0x1, 0x0] var_off=(0x0, 0x0)
WARNING: CPU: 0 PID: 92 at kernel/bpf/verifier.c:2731 reg_bounds_sanity_check+0x163/0x220
Hardware name: QEMU Ubuntu 24.04 PC (i440FX + PIIX, 1996), BIOS 1.16.3-debian-1.16.3-2 04/01/2014
RIP: 0010:reg_bounds_sanity_check+0x163/0x220
Call Trace:
 <TASK>
 reg_set_min_max+0xf7/0x1d0
 check_cond_jmp_op+0x57b/0x1730
 ? print_bpf_insn+0x3d5/0xa50
 do_check_common+0x33ac/0x33c0
 ...

The root cause is in regs_refine_cond_op() where BPF_JLT/BPF_JSLT operations
adjust both min/max bounds on the same register, causing invalid bounds.

Since comparing a register with itself should not change its bounds (the
comparison result is always known: r0 == r0 is always true, r0 < r0 is
always false), the bounds adjustment is unnecessary.

Fix this by:
1. Enhance is_branch_taken() and is_scalar_branch_taken() to properly
   handle branch direction computation for same register comparisons
   across all BPF jump operations
2. For unknown branch directions (e.g., BPF_JSET), add early return in
   reg_set_min_max() to avoid bounds adjustment on the same register

The fix ensures that unnecessary bounds adjustments are skipped, preventing
the verifier bug while maintaining correct branch direction analysis.

Reported-by: Kaiyan Mei <M202472210@hust.edu.cn>
Reported-by: Yinhao Hu <dddddd@hust.edu.cn>
Closes: https://lore.kernel.org/all/1881f0f5.300df.199f2576a01.Coremail.kaiyanm@hust.edu.cn/
Fixes: 0df1a55afa83 ("bpf: Warn on internal verifier errors")
Signed-off-by: KaFai Wan <kafai.wan@linux.dev>
---
 kernel/bpf/verifier.c | 32 ++++++++++++++++++++++++++++++++
 1 file changed, 32 insertions(+)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 6d175849e57a..653fa96ed0df 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -16037,6 +16037,12 @@ static int is_scalar_branch_taken(struct bpf_reg_state *reg1, struct bpf_reg_sta
 		}
 		break;
 	case BPF_JSET:
+		if (reg1 == reg2) {
+			if (tnum_is_const(t1))
+				return t1.value != 0;
+			else
+				return (smin1 <= 0 && smax1 >= 0) ? -1 : 1;
+		}
 		if (!is_reg_const(reg2, is_jmp32)) {
 			swap(reg1, reg2);
 			swap(t1, t2);
@@ -16172,6 +16178,25 @@ static int is_pkt_ptr_branch_taken(struct bpf_reg_state *dst_reg,
 static int is_branch_taken(struct bpf_reg_state *reg1, struct bpf_reg_state *reg2,
 			   u8 opcode, bool is_jmp32)
 {
+	if (reg1 == reg2) {
+		switch (opcode) {
+		case BPF_JGE:
+		case BPF_JLE:
+		case BPF_JSGE:
+		case BPF_JSLE:
+		case BPF_JEQ:
+			return 1;
+		case BPF_JGT:
+		case BPF_JLT:
+		case BPF_JSGT:
+		case BPF_JSLT:
+		case BPF_JNE:
+			return 0;
+		default:
+			break;
+		}
+	}
+
 	if (reg_is_pkt_pointer_any(reg1) && reg_is_pkt_pointer_any(reg2) && !is_jmp32)
 		return is_pkt_ptr_branch_taken(reg1, reg2, opcode);
 
@@ -16429,6 +16454,13 @@ static int reg_set_min_max(struct bpf_verifier_env *env,
 	if (false_reg1->type != SCALAR_VALUE || false_reg2->type != SCALAR_VALUE)
 		return 0;
 
+	/* We compute branch direction for same registers in is_branch_taken() and
+	 * is_scalar_branch_taken(). For unknown branch directions (e.g., BPF_JSET)
+	 * on the same registers, we don't need to adjusts the min/max values.
+	 */
+	if (false_reg1 == false_reg2)
+		return 0;
+
 	/* fallthrough (FALSE) branch */
 	regs_refine_cond_op(false_reg1, false_reg2, rev_opcode(opcode), is_jmp32);
 	reg_bounds_sync(false_reg1);
-- 
2.43.0
Re: [PATCH bpf-next v2 1/2] bpf: Skip bounds adjustment for conditional jumps on same register
Posted by Eduard Zingerman 3 months, 1 week ago 
On Sat, 2025-10-25 at 13:30 +0800, KaFai Wan wrote:
> When conditional jumps are performed on the same register (e.g., r0 <= r0,
> r0 > r0, r0 < r0) where the register holds a scalar with range, the verifier
> incorrectly attempts to adjust the register's min/max bounds. This leads to
> invalid range bounds and triggers a BUG warning:
> 
> verifier bug: REG INVARIANTS VIOLATION (true_reg1): range bounds violation u64=[0x1, 0x0] s64=[0x1, 0x0] u32=[0x1, 0x0] s32=[0x1, 0x0] var_off=(0x0, 0x0)
> WARNING: CPU: 0 PID: 92 at kernel/bpf/verifier.c:2731 reg_bounds_sanity_check+0x163/0x220
> Hardware name: QEMU Ubuntu 24.04 PC (i440FX + PIIX, 1996), BIOS 1.16.3-debian-1.16.3-2 04/01/2014
> RIP: 0010:reg_bounds_sanity_check+0x163/0x220
> Call Trace:
>  <TASK>
>  reg_set_min_max+0xf7/0x1d0
>  check_cond_jmp_op+0x57b/0x1730
>  ? print_bpf_insn+0x3d5/0xa50
>  do_check_common+0x33ac/0x33c0
>  ...
> 
> The root cause is in regs_refine_cond_op() where BPF_JLT/BPF_JSLT operations
> adjust both min/max bounds on the same register, causing invalid bounds.
> 
> Since comparing a register with itself should not change its bounds (the
> comparison result is always known: r0 == r0 is always true, r0 < r0 is
> always false), the bounds adjustment is unnecessary.
> 
> Fix this by:
> 1. Enhance is_branch_taken() and is_scalar_branch_taken() to properly
>    handle branch direction computation for same register comparisons
>    across all BPF jump operations
> 2. For unknown branch directions (e.g., BPF_JSET), add early return in
>    reg_set_min_max() to avoid bounds adjustment on the same register
> 
> The fix ensures that unnecessary bounds adjustments are skipped, preventing
> the verifier bug while maintaining correct branch direction analysis.
> 
> Reported-by: Kaiyan Mei <M202472210@hust.edu.cn>
> Reported-by: Yinhao Hu <dddddd@hust.edu.cn>
> Closes: https://lore.kernel.org/all/1881f0f5.300df.199f2576a01.Coremail.kaiyanm@hust.edu.cn/
> Fixes: 0df1a55afa83 ("bpf: Warn on internal verifier errors")
> Signed-off-by: KaFai Wan <kafai.wan@linux.dev>
> ---
>  kernel/bpf/verifier.c | 32 ++++++++++++++++++++++++++++++++
>  1 file changed, 32 insertions(+)
> 
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 6d175849e57a..653fa96ed0df 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -16037,6 +16037,12 @@ static int is_scalar_branch_taken(struct bpf_reg_state *reg1, struct bpf_reg_sta
>  		}
>  		break;
>  	case BPF_JSET:
> +		if (reg1 == reg2) {
> +			if (tnum_is_const(t1))
> +				return t1.value != 0;
> +			else
> +				return (smin1 <= 0 && smax1 >= 0) ? -1 : 1;
> +		}

I think this logic is fine, but it needs tests for multiple cases.

>  		if (!is_reg_const(reg2, is_jmp32)) {
>  			swap(reg1, reg2);
>  			swap(t1, t2);
> @@ -16172,6 +16178,25 @@ static int is_pkt_ptr_branch_taken(struct bpf_reg_state *dst_reg,
>  static int is_branch_taken(struct bpf_reg_state *reg1, struct bpf_reg_state *reg2,
>  			   u8 opcode, bool is_jmp32)
>  {
> +	if (reg1 == reg2) {
> +		switch (opcode) {
> +		case BPF_JGE:
> +		case BPF_JLE:
> +		case BPF_JSGE:
> +		case BPF_JSLE:
> +		case BPF_JEQ:
> +			return 1;
> +		case BPF_JGT:
> +		case BPF_JLT:
> +		case BPF_JSGT:
> +		case BPF_JSLT:
> +		case BPF_JNE:
> +			return 0;
> +		default:
> +			break;
> +		}
> +	}
> +

I think Alexei was against my suggestion to put it in
is_branch_taken() and preferred is_scalar_branch_taken() instead.

>  	if (reg_is_pkt_pointer_any(reg1) && reg_is_pkt_pointer_any(reg2) && !is_jmp32)
>  		return is_pkt_ptr_branch_taken(reg1, reg2, opcode);
>  
> @@ -16429,6 +16454,13 @@ static int reg_set_min_max(struct bpf_verifier_env *env,
>  	if (false_reg1->type != SCALAR_VALUE || false_reg2->type != SCALAR_VALUE)
>  		return 0;
>  
> +	/* We compute branch direction for same registers in is_branch_taken() and
> +	 * is_scalar_branch_taken(). For unknown branch directions (e.g., BPF_JSET)
> +	 * on the same registers, we don't need to adjusts the min/max values.
> +	 */
> +	if (false_reg1 == false_reg2)
> +		return 0;
> +
>  	/* fallthrough (FALSE) branch */
>  	regs_refine_cond_op(false_reg1, false_reg2, rev_opcode(opcode), is_jmp32);
>  	reg_bounds_sync(false_reg1);
Re: [PATCH bpf-next v2 1/2] bpf: Skip bounds adjustment for conditional jumps on same register
Posted by KaFai Wan 3 months, 1 week ago 
On Mon, 2025-10-27 at 13:09 -0700, Eduard Zingerman wrote:
> On Sat, 2025-10-25 at 13:30 +0800, KaFai Wan wrote:
> > When conditional jumps are performed on the same register (e.g., r0 <= r0,
> > r0 > r0, r0 < r0) where the register holds a scalar with range, the verifier
> > incorrectly attempts to adjust the register's min/max bounds. This leads to
> > invalid range bounds and triggers a BUG warning:
> > 
> > verifier bug: REG INVARIANTS VIOLATION (true_reg1): range bounds violation u64=[0x1, 0x0]
> > s64=[0x1, 0x0] u32=[0x1, 0x0] s32=[0x1, 0x0] var_off=(0x0, 0x0)
> > WARNING: CPU: 0 PID: 92 at kernel/bpf/verifier.c:2731 reg_bounds_sanity_check+0x163/0x220
> > Hardware name: QEMU Ubuntu 24.04 PC (i440FX + PIIX, 1996), BIOS 1.16.3-debian-1.16.3-2
> > 04/01/2014
> > RIP: 0010:reg_bounds_sanity_check+0x163/0x220
> > Call Trace:
> >  <TASK>
> >  reg_set_min_max+0xf7/0x1d0
> >  check_cond_jmp_op+0x57b/0x1730
> >  ? print_bpf_insn+0x3d5/0xa50
> >  do_check_common+0x33ac/0x33c0
> >  ...
> > 
> > The root cause is in regs_refine_cond_op() where BPF_JLT/BPF_JSLT operations
> > adjust both min/max bounds on the same register, causing invalid bounds.
> > 
> > Since comparing a register with itself should not change its bounds (the
> > comparison result is always known: r0 == r0 is always true, r0 < r0 is
> > always false), the bounds adjustment is unnecessary.
> > 
> > Fix this by:
> > 1. Enhance is_branch_taken() and is_scalar_branch_taken() to properly
> >    handle branch direction computation for same register comparisons
> >    across all BPF jump operations
> > 2. For unknown branch directions (e.g., BPF_JSET), add early return in
> >    reg_set_min_max() to avoid bounds adjustment on the same register
> > 
> > The fix ensures that unnecessary bounds adjustments are skipped, preventing
> > the verifier bug while maintaining correct branch direction analysis.
> > 
> > Reported-by: Kaiyan Mei <M202472210@hust.edu.cn>
> > Reported-by: Yinhao Hu <dddddd@hust.edu.cn>
> > Closes: https://lore.kernel.org/all/1881f0f5.300df.199f2576a01.Coremail.kaiyanm@hust.edu.cn/
> > Fixes: 0df1a55afa83 ("bpf: Warn on internal verifier errors")
> > Signed-off-by: KaFai Wan <kafai.wan@linux.dev>
> > ---
> >  kernel/bpf/verifier.c | 32 ++++++++++++++++++++++++++++++++
> >  1 file changed, 32 insertions(+)
> > 
> > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> > index 6d175849e57a..653fa96ed0df 100644
> > --- a/kernel/bpf/verifier.c
> > +++ b/kernel/bpf/verifier.c
> > @@ -16037,6 +16037,12 @@ static int is_scalar_branch_taken(struct bpf_reg_state *reg1, struct
> > bpf_reg_sta
> >  		}
> >  		break;
> >  	case BPF_JSET:
> > +		if (reg1 == reg2) {
> > +			if (tnum_is_const(t1))
> > +				return t1.value != 0;
> > +			else
> > +				return (smin1 <= 0 && smax1 >= 0) ? -1 : 1;
> > +		}
> 
> I think this logic is fine, but it needs tests for multiple cases.
> 
ok, I'll add tests for that.

> >  		if (!is_reg_const(reg2, is_jmp32)) {
> >  			swap(reg1, reg2);
> >  			swap(t1, t2);
> > @@ -16172,6 +16178,25 @@ static int is_pkt_ptr_branch_taken(struct bpf_reg_state *dst_reg,
> >  static int is_branch_taken(struct bpf_reg_state *reg1, struct bpf_reg_state *reg2,
> >  			   u8 opcode, bool is_jmp32)
> >  {
> > +	if (reg1 == reg2) {
> > +		switch (opcode) {
> > +		case BPF_JGE:
> > +		case BPF_JLE:
> > +		case BPF_JSGE:
> > +		case BPF_JSLE:
> > +		case BPF_JEQ:
> > +			return 1;
> > +		case BPF_JGT:
> > +		case BPF_JLT:
> > +		case BPF_JSGT:
> > +		case BPF_JSLT:
> > +		case BPF_JNE:
> > +			return 0;
> > +		default:
> > +			break;
> > +		}
> > +	}
> > +
> 
> I think Alexei was against my suggestion to put it in
> is_branch_taken() and preferred is_scalar_branch_taken() instead.
> 
Hmm, I misunderstood that. If put in is_scalar_branch_taken() then just for scalar cases,
just confirm that.

> >  	if (reg_is_pkt_pointer_any(reg1) && reg_is_pkt_pointer_any(reg2) && !is_jmp32)
> >  		return is_pkt_ptr_branch_taken(reg1, reg2, opcode);
> >  
> > @@ -16429,6 +16454,13 @@ static int reg_set_min_max(struct bpf_verifier_env *env,
> >  	if (false_reg1->type != SCALAR_VALUE || false_reg2->type != SCALAR_VALUE)
> >  		return 0;
> >  
> > +	/* We compute branch direction for same registers in is_branch_taken() and
> > +	 * is_scalar_branch_taken(). For unknown branch directions (e.g., BPF_JSET)
> > +	 * on the same registers, we don't need to adjusts the min/max values.
> > +	 */
> > +	if (false_reg1 == false_reg2)
> > +		return 0;
> > +
> >  	/* fallthrough (FALSE) branch */
> >  	regs_refine_cond_op(false_reg1, false_reg2, rev_opcode(opcode), is_jmp32);
> >  	reg_bounds_sync(false_reg1);

-- 
Thanks,
KaFai

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