[PATCH bpf-next 0/2] bpf: Fix tnum_overlap to check for zero mask first

[PATCH bpf-next 0/2] bpf: Fix tnum_overlap to check for zero mask first

[KaFai Wan]

This small patchset is about avoid verifier bug warning when tnum_overlap()
is called with zero mask.

---
KaFai Wan (2):
  bpf: Fix tnum_overlap to check for zero mask first
  selftests/bpf: Range analysis test case for JEQ

 kernel/bpf/tnum.c                             |  2 ++
 .../selftests/bpf/progs/verifier_bounds.c     | 23 +++++++++++++++++++
 2 files changed, 25 insertions(+)

-- 
2.43.0

[PATCH bpf-next 1/2] bpf: Fix tnum_overlap to check for zero mask first

[KaFai Wan]

Syzbot reported a kernel warning due to a range invariant violation in
the BPF verifier. The issue occurs when tnum_overlap() fails to detect
that two tnums don't have any overlapping bits.

The problematic BPF program:
   0: call bpf_get_prandom_u32
   1: r6 = r0
   2: r6 &= 0xFFFFFFFFFFFFFFF0
   3: r7 = r0
   4: r7 &= 0x07
   5: r7 -= 0xFF
   6: if r6 == r7 goto <exit>

After instruction 5, R7 has the range:
   R7: u64=[0xffffffffffffff01, 0xffffffffffffff08] var_off=(0xffffffffffffff00; 0xf)

R6 and R7 don't overlap since they have no agreeing bits. However,
is_branch_taken() fails to recognize this, causing the verifier to
refine register bounds and end up with inconsistent bounds:

   6: if r6 == r7 goto <exit>
   R6: u64=[0xffffffffffffff01, 0xffffffffffffff00] var_off=(0xffffffffffffff00, 0x0)
   R7: u64=[0xffffffffffffff01, 0xffffffffffffff00] var_off=(0xffffffffffffff00, 0x0)

The root cause is that tnum_overlap() doesn't properly handle the case
where the masks have no overlapping bits.

Fix this by adding an early check for zero mask intersection in tnum_overlap().

Reported-by: syzbot+c950cc277150935cc0b5@syzkaller.appspotmail.com
Fixes: f41345f47fb2 ("bpf: Use tnums for JEQ/JNE is_branch_taken logic")
Signed-off-by: KaFai Wan <kafai.wan@linux.dev>
---
 kernel/bpf/tnum.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/kernel/bpf/tnum.c b/kernel/bpf/tnum.c
index f8e70e9c3998..af2f38b4f840 100644
--- a/kernel/bpf/tnum.c
+++ b/kernel/bpf/tnum.c
@@ -163,6 +163,8 @@ bool tnum_overlap(struct tnum a, struct tnum b)
 {
 	u64 mu;
 
+	if ((a.mask & b.mask) == 0)
+		return false;
 	mu = ~a.mask & ~b.mask;
 	return (a.value & mu) == (b.value & mu);
 }
-- 
2.43.0

Re: [PATCH bpf-next 1/2] bpf: Fix tnum_overlap to check for zero mask first

[KaFai Wan]

Sorry, this patch is wrong, please ignore.

On Mon, 2025-10-27 at 00:38 +0800, KaFai Wan wrote:
> Syzbot reported a kernel warning due to a range invariant violation in
> the BPF verifier. The issue occurs when tnum_overlap() fails to detect
> that two tnums don't have any overlapping bits.
> 
> The problematic BPF program:
>    0: call bpf_get_prandom_u32
>    1: r6 = r0
>    2: r6 &= 0xFFFFFFFFFFFFFFF0
>    3: r7 = r0
>    4: r7 &= 0x07
>    5: r7 -= 0xFF
>    6: if r6 == r7 goto <exit>
> 
> After instruction 5, R7 has the range:
>    R7: u64=[0xffffffffffffff01, 0xffffffffffffff08] var_off=(0xffffffffffffff00; 0xf)
> 
> R6 and R7 don't overlap since they have no agreeing bits. However,
> is_branch_taken() fails to recognize this, causing the verifier to
> refine register bounds and end up with inconsistent bounds:
> 
>    6: if r6 == r7 goto <exit>
>    R6: u64=[0xffffffffffffff01, 0xffffffffffffff00] var_off=(0xffffffffffffff00, 0x0)
>    R7: u64=[0xffffffffffffff01, 0xffffffffffffff00] var_off=(0xffffffffffffff00, 0x0)
> 
> The root cause is that tnum_overlap() doesn't properly handle the case
> where the masks have no overlapping bits.
> 
> Fix this by adding an early check for zero mask intersection in tnum_overlap().
> 
> Reported-by: syzbot+c950cc277150935cc0b5@syzkaller.appspotmail.com
> Fixes: f41345f47fb2 ("bpf: Use tnums for JEQ/JNE is_branch_taken logic")
> Signed-off-by: KaFai Wan <kafai.wan@linux.dev>
> ---
>  kernel/bpf/tnum.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/kernel/bpf/tnum.c b/kernel/bpf/tnum.c
> index f8e70e9c3998..af2f38b4f840 100644
> --- a/kernel/bpf/tnum.c
> +++ b/kernel/bpf/tnum.c
> @@ -163,6 +163,8 @@ bool tnum_overlap(struct tnum a, struct tnum b)
>  {
>  	u64 mu;
>  
> +	if ((a.mask & b.mask) == 0)
> +		return false;
>  	mu = ~a.mask & ~b.mask;
>  	return (a.value & mu) == (b.value & mu);
>  }

-- 
Thanks,
KaFai

[PATCH bpf-next 2/2] selftests/bpf: Range analysis test case for JEQ

[KaFai Wan]

This patch adds coverage for the warning detected by syzkaller and fixed
in the previous patch. Without the previous patch, this test fails with:

  verifier bug: REG INVARIANTS VIOLATION (true_reg1): range bounds
  violation u64=[0xffffffffffffff01, 0xffffffffffffff00]
  s64=[0xffffffffffffff01, 0xffffffffffffff00]
  u32=[0xffffff01, 0xffffff00] s32=[0xffffff00, 0xffffff00]
  var_off=(0xffffffffffffff00, 0x0)
  verifier bug: REG INVARIANTS VIOLATION (true_reg2): range bounds
  violation u64=[0xffffffffffffff01, 0xffffffffffffff00]
  s64=[0xffffffffffffff01, 0xffffffffffffff00]
  u32=[0xffffff01, 0xffffff00] s32=[0xffffff01, 0xffffff00]
  var_off=(0xffffffffffffff00, 0x0)

Signed-off-by: KaFai Wan <kafai.wan@linux.dev>
---
 .../selftests/bpf/progs/verifier_bounds.c     | 23 +++++++++++++++++++
 1 file changed, 23 insertions(+)

diff --git a/tools/testing/selftests/bpf/progs/verifier_bounds.c b/tools/testing/selftests/bpf/progs/verifier_bounds.c
index 0a72e0228ea9..304ab5a07a3b 100644
--- a/tools/testing/selftests/bpf/progs/verifier_bounds.c
+++ b/tools/testing/selftests/bpf/progs/verifier_bounds.c
@@ -1550,6 +1550,29 @@ l0_%=:	r0 = 0;				\
 	: __clobber_all);
 }
 
+SEC("socket")
+__description("dead branch on jeq, does not result in invariants violation error")
+__success __log_level(2)
+__retval(0) __flag(BPF_F_TEST_REG_INVARIANTS)
+__naked void jeq_range_analysis(void)
+{
+	asm volatile ("			\
+	call %[bpf_get_prandom_u32];	\
+	r6 = r0;			\
+	r6 &= 0xFFFFFFFFFFFFFFF0;	\
+	r7 = r0;			\
+	r7 &= 0x07;			\
+	r7 -= 0xFF;			\
+	if r6 == r7 goto l1_%=;		\
+l0_%=:  r0 = 0;				\
+	exit;				\
+l1_%=:  r0 = 1;				\
+	exit;				\
+"	:
+	: __imm(bpf_get_prandom_u32)
+	: __clobber_all);
+}
+
 /* This test covers the bounds deduction on 64bits when the s64 and u64 ranges
  * overlap on the negative side. At instruction 7, the ranges look as follows:
  *
-- 
2.43.0