[PATCH bpf-next v2 1/2] bpf: Forget ranges when refining tnum after JSET

[PATCH bpf-next v2 1/2] bpf: Forget ranges when refining tnum after JSET

[Paul Chaignon]

Syzbot reported a kernel warning due to a range invariant violation on
the following BPF program.

  0: call bpf_get_netns_cookie
  1: if r0 == 0 goto <exit>
  2: if r0 & Oxffffffff goto <exit>

The issue is on the path where we fall through both jumps.

That path is unreachable at runtime: after insn 1, we know r0 != 0, but
with the sign extension on the jset, we would only fallthrough insn 2
if r0 == 0. Unfortunately, is_branch_taken() isn't currently able to
figure this out, so the verifier walks all branches. The verifier then
refines the register bounds using the second condition and we end
up with inconsistent bounds on this unreachable path:

  1: if r0 == 0 goto <exit>
    r0: u64=[0x1, 0xffffffffffffffff] var_off=(0, 0xffffffffffffffff)
  2: if r0 & 0xffffffff goto <exit>
    r0 before reg_bounds_sync: u64=[0x1, 0xffffffffffffffff] var_off=(0, 0)
    r0 after reg_bounds_sync:  u64=[0x1, 0] var_off=(0, 0)

Improving the range refinement for JSET to cover all cases is tricky. We
also don't expect many users to rely on JSET given LLVM doesn't generate
those instructions. So instead of improving the range refinement for
JSETs, Eduard suggested we forget the ranges whenever we're narrowing
tnums after a JSET. This patch implements that approach.

Reported-by: syzbot+c711ce17dd78e5d4fdcf@syzkaller.appspotmail.com
Suggested-by: Eduard Zingerman <eddyz87@gmail.com>
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Paul Chaignon <paul.chaignon@gmail.com>
---
 kernel/bpf/verifier.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 53007182b46b..e2fcea860755 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -16208,6 +16208,10 @@ static void regs_refine_cond_op(struct bpf_reg_state *reg1, struct bpf_reg_state
 		if (!is_reg_const(reg2, is_jmp32))
 			break;
 		val = reg_const_value(reg2, is_jmp32);
+		/* Forget the ranges before narrowing tnums, to avoid invariant
+		 * violations if we're on a dead branch.
+		 */
+		__mark_reg_unbounded(reg1);
 		if (is_jmp32) {
 			t = tnum_and(tnum_subreg(reg1->var_off), tnum_const(~val));
 			reg1->var_off = tnum_with_subreg(reg1->var_off, t);
-- 
2.43.0

[PATCH bpf-next v2 2/2] selftests/bpf: Range analysis test case for JSET

[Paul Chaignon]

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 (false_reg1): range bounds
  violation u64=[0x0, 0x0] s64=[0x0, 0x0] u32=[0x1, 0x0] s32=[0x0, 0x0]
  var_off=(0x0, 0x0)(1)

Signed-off-by: Paul Chaignon <paul.chaignon@gmail.com>
---
Changes in v2:
  - As suggested by Yonghong, revert use of __imm_insn for the jset
    insn since newer LLVM versions support it.

 .../selftests/bpf/progs/verifier_bounds.c      | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/tools/testing/selftests/bpf/progs/verifier_bounds.c b/tools/testing/selftests/bpf/progs/verifier_bounds.c
index 6f986ae5085e..63b533ca4933 100644
--- a/tools/testing/selftests/bpf/progs/verifier_bounds.c
+++ b/tools/testing/selftests/bpf/progs/verifier_bounds.c
@@ -2,6 +2,7 @@
 /* Converted from tools/testing/selftests/bpf/verifier/bounds.c */
 
 #include <linux/bpf.h>
+#include <../../../include/linux/filter.h>
 #include <bpf/bpf_helpers.h>
 #include "bpf_misc.h"
 
@@ -1532,4 +1533,21 @@ __naked void sub32_partial_overflow(void)
 	: __clobber_all);
 }
 
+SEC("socket")
+__description("dead branch on jset, does not result in invariants violation error")
+__success __log_level(2)
+__retval(0) __flag(BPF_F_TEST_REG_INVARIANTS)
+__naked void jset_range_analysis(void)
+{
+	asm volatile ("			\
+	call %[bpf_get_netns_cookie];	\
+	if r0 == 0 goto l0_%=;		\
+	if r0 & 0xffffffff goto +0; 	\
+l0_%=:	r0 = 0;				\
+	exit;				\
+"	:
+	: __imm(bpf_get_netns_cookie)
+	: __clobber_all);
+}
+
 char _license[] SEC("license") = "GPL";
-- 
2.43.0

Re: [PATCH bpf-next v2 2/2] selftests/bpf: Range analysis test case for JSET

[Yonghong Song]

On 7/10/25 11:21 AM, Paul Chaignon wrote:
> 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 (false_reg1): range bounds
>    violation u64=[0x0, 0x0] s64=[0x0, 0x0] u32=[0x1, 0x0] s32=[0x0, 0x0]
>    var_off=(0x0, 0x0)(1)
>
> Signed-off-by: Paul Chaignon <paul.chaignon@gmail.com>

Acked-by: Yonghong Song <yonghong.song@linux.dev>

Re: [PATCH bpf-next v2 1/2] bpf: Forget ranges when refining tnum after JSET

[patchwork-bot+netdevbpf]

Hello:

This series was applied to bpf/bpf-next.git (master)
by Alexei Starovoitov <ast@kernel.org>:

On Thu, 10 Jul 2025 20:20:53 +0200 you wrote:
> Syzbot reported a kernel warning due to a range invariant violation on
> the following BPF program.
> 
>   0: call bpf_get_netns_cookie
>   1: if r0 == 0 goto <exit>
>   2: if r0 & Oxffffffff goto <exit>
> 
> [...]

Here is the summary with links:
  - [bpf-next,v2,1/2] bpf: Forget ranges when refining tnum after JSET
    https://git.kernel.org/bpf/bpf-next/c/6279846b9b25
  - [bpf-next,v2,2/2] selftests/bpf: Range analysis test case for JSET
    https://git.kernel.org/bpf/bpf-next/c/d81526a6ebff

You are awesome, thank you!
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