[PATCH bpf 0/2] bpf, verifier: fix PTR_TO_FLOW_KEYS constant-offset OOB

[PATCH bpf 0/2] bpf, verifier: fix PTR_TO_FLOW_KEYS constant-offset OOB

[Nuiqi Gui]

A constant offset added to a PTR_TO_FLOW_KEYS register lands in
reg->var_off, but check_flow_keys_access() bounds-checks only insn->off
and never folds reg->var_off.value.  A BPF_PROG_TYPE_FLOW_DISSECTOR
program can therefore do "flow_keys += 0x1000; *(flow_keys + 0)" and have
it accepted, then read/write kernel stack past struct bpf_flow_keys at
runtime.  Patch 1 folds reg->var_off.value into the offset (and rejects
non-constant offsets), mirroring check_ctx_access(); patch 2 adds verifier
selftests.

This is a regression introduced in the 7.1 development cycle by commit
022ac0750883 ("bpf: use reg->var_off instead of reg->off for pointers"),
which moved the constant offset from reg->off (folded generically before
022ac0750883) into reg->var_off without updating the flow_keys path.  No
released kernel is affected: v7.0.x rejects the program above, and the bug
reproduces only on v7.1-rc1..rc5, so no stable backport is needed.

It was first reported privately to security@kernel.org; per their guidance
it is handled in the open as a normal regression fix.  Found by manual
verifier audit and confirmed dynamically in a disposable QEMU/KVM guest:
the load above is accepted, a runtime read leaked a kernel-stack pointer
0x1000 past bpf_flow_keys, and a runtime write of a marker faulted the
guest in net_rx_action.

An alternative -- forbidding pointer arithmetic on PTR_TO_FLOW_KEYS
outright by dropping "if (known) break;" in adjust_ptr_min_max_vals() --
was rejected because v7.0.x accepted (and correctly bounds-checked)
constant arithmetic on the keys pointer; restoring the fold preserves that
behaviour while closing the divergence.

Nuiqi Gui (2):
  bpf, verifier: fold reg->var_off into PTR_TO_FLOW_KEYS bounds check
  selftests/bpf: add tests for PTR_TO_FLOW_KEYS constant offset bounds

 kernel/bpf/verifier.c                         | 19 ++++-
 .../selftests/bpf/prog_tests/verifier.c       |  2 +
 .../selftests/bpf/progs/verifier_flow_keys.c  | 77 +++++++++++++++++++
 3 files changed, 95 insertions(+), 3 deletions(-)
 create mode 100644 tools/testing/selftests/bpf/progs/verifier_flow_keys.c


base-commit: e7ae89a0c97ce2b68b0983cd01eda67cf373517d
-- 
2.34.1

[PATCH bpf 1/2] bpf, verifier: fold reg->var_off into PTR_TO_FLOW_KEYS bounds check

[Nuiqi Gui]

Constant pointer arithmetic on a PTR_TO_FLOW_KEYS register lands the
constant in reg->var_off (e.g. flow_keys(imm=4096)), but the
PTR_TO_FLOW_KEYS path in check_mem_access() passes only insn->off to
check_flow_keys_access() and never folds reg->var_off.value.  The
verifier therefore accepts an access that, at runtime, dereferences past
struct bpf_flow_keys -- a verifier/runtime divergence that yields an
out-of-bounds read and write of kernel stack memory.

Commit 022ac0750883 ("bpf: use reg->var_off instead of reg->off for
pointers") removed the generic "off += reg->off" that check_mem_access()
applied before the per-type dispatch and replaced it with per-path
folding of reg->var_off.value (for example the ctx path now folds the
register offset via check_ctx_access()).  The PTR_TO_FLOW_KEYS path was
not given the equivalent fold, so a constant offset that used to be
folded and rejected is now silently accepted:

  before 022ac0750883: the offset stays in reg->off and is folded
    generically, so the access is checked with off=4096 and rejected.
  after  022ac0750883: the offset lands in reg->var_off, the flow_keys
    path checks off=0 and accepts; at runtime the access dereferences
    base + 0x1000.

For a BPF_PROG_TYPE_FLOW_DISSECTOR program the following is accepted:

  r2 = *(u64 *)(r1 + 144)   ; R2=flow_keys (PTR_TO_FLOW_KEYS)
  r2 += 0x1000              ; R2=flow_keys(imm=4096), accepted
  r0 = *(u64 *)(r2 + 0)     ; accepted, var_off.value=0x1000 ignored

while the equivalent insn->off form

  r0 = *(u64 *)(r2 + 0x1000)

has the same effective offset but is correctly rejected with
"invalid access to flow keys off=4096 size=8", which isolates the defect
to the missing var_off fold.  Once attached as a flow dissector, the
accepted program reads kernel stack past struct bpf_flow_keys (a
kernel-stack / KASLR information leak) and can likewise write past it,
corrupting kernel memory.

Fix it by folding reg->var_off.value into the offset before the bounds
check and rejecting non-constant offsets, mirroring the other pointer
types (e.g. check_ctx_access()).

No released kernel is affected; the regression is confined to the 7.1
development cycle (reproduced on v7.1-rc1..rc5), and v7.0.x rejects the
program above.

Fixes: 022ac0750883 ("bpf: use reg->var_off instead of reg->off for pointers")
Signed-off-by: Nuiqi Gui <gnq25@mails.tsinghua.edu.cn>
---
 kernel/bpf/verifier.c | 19 ++++++++++++++++---
 1 file changed, 16 insertions(+), 3 deletions(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 7fb88e1cd7c4..e8a8dd89af06 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -4997,9 +4997,22 @@ static int check_ctx_access(struct bpf_verifier_env *env, int insn_idx, u32 regn
 	return err;
 }
 
-static int check_flow_keys_access(struct bpf_verifier_env *env, int off,
-				  int size)
+static int check_flow_keys_access(struct bpf_verifier_env *env, u32 regno,
+				  int off, int size)
 {
+	struct bpf_reg_state *reg = reg_state(env, regno);
+
+	/* Only a constant offset is allowed here; fold it into off. */
+	if (!tnum_is_const(reg->var_off)) {
+		char tn_buf[48];
+
+		tnum_strn(tn_buf, sizeof(tn_buf), reg->var_off);
+		verbose(env, "R%d invalid variable offset to flow keys: off=%d, var_off=%s\n",
+			regno, off, tn_buf);
+		return -EACCES;
+	}
+	off += reg->var_off.value;
+
 	if (size < 0 || off < 0 ||
 	    (u64)off + size > sizeof(struct bpf_flow_keys)) {
 		verbose(env, "invalid access to flow keys off=%d size=%d\n",
@@ -6514,7 +6527,7 @@ static int check_mem_access(struct bpf_verifier_env *env, int insn_idx, u32 regn
 			return -EACCES;
 		}
 
-		err = check_flow_keys_access(env, off, size);
+		err = check_flow_keys_access(env, regno, off, size);
 		if (!err && t == BPF_READ && value_regno >= 0)
 			mark_reg_unknown(env, regs, value_regno);
 	} else if (type_is_sk_pointer(reg->type)) {
-- 
2.34.1

Re: [PATCH bpf 1/2] bpf, verifier: fold reg->var_off into PTR_TO_FLOW_KEYS bounds check

[sashiko-bot]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:

Pre-existing issues:
- [Critical] Negative variable offsets on PTR_TO_BUF and PTR_TO_TP_BUFFER completely bypass memory bounds tracking, permitting out-of-bounds reads/writes at runtime.
--

commit 2118c968df54a167a0c832db86595b84ed0a8c71
Author: Nuiqi Gui <gnq25@mails.tsinghua.edu.cn>

bpf, verifier: fold reg->var_off into PTR_TO_FLOW_KEYS bounds check

This commit fixes a bounds checking bypass for PTR_TO_FLOW_KEYS by folding
a constant reg->var_off into the offset before validation. This prevents
out-of-bounds reads and writes to kernel stack memory.

> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -4997,9 +4997,22 @@ static int check_ctx_access(struct bpf_verifier_env *env, int insn_idx, u32 regn
>  	return err;
>  }
>  
> -static int check_flow_keys_access(struct bpf_verifier_env *env, int off,
> -				  int size)
> +static int check_flow_keys_access(struct bpf_verifier_env *env, u32 regno,
> +				  int off, int size)
>  {
> +	struct bpf_reg_state *reg = reg_state(env, regno);
> +
> +	/* Only a constant offset is allowed here; fold it into off. */
> +	if (!tnum_is_const(reg->var_off)) {
> +		char tn_buf[48];
> +
> +		tnum_strn(tn_buf, sizeof(tn_buf), reg->var_off);
> +		verbose(env, "R%d invalid variable offset to flow keys: off=%d, var_off=%s\n",
> +			regno, off, tn_buf);
> +		return -EACCES;
> +	}
> +	off += reg->var_off.value;
> +
>  	if (size < 0 || off < 0 ||
>  	    (u64)off + size > sizeof(struct bpf_flow_keys)) {
>  		verbose(env, "invalid access to flow keys off=%d size=%d\n",

[Severity: Critical]
This isn't a bug introduced by this patch, but while looking at how we
validate reg->var_off.value here, do check_buffer_access() and
check_tp_buffer_access() have a vulnerability with negative variable offsets?

In __check_buffer_access(), we reject non-constant variable offsets, but we
don't seem to verify if reg->var_off.value or reg->smin_value is negative:

kernel/bpf/verifier.c:__check_buffer_access() {
    ...
    if (!tnum_is_const(reg->var_off)) {
        ...
    }
    ...
}

Later in check_buffer_access(), this gets added to a u32 *max_access:

kernel/bpf/verifier.c:check_buffer_access() {
    ...
    *max_access = max(reg->var_off.value + off + size, *max_access);
    ...
}

If reg->var_off.value is a large negative 64-bit constant whose lower 32
bits evaluate to a small positive number (e.g., -4294967296, which is
0xFFFFFFFF00000000ULL), does reg->var_off.value + off + size truncate when
assigned to the u32 *max_access?

If it truncates to a small value, it seems like it could bypass the bounds
check, making a massively out-of-bounds access appear perfectly valid to
the verifier.

It appears check_tp_buffer_access() might apply the identical bounds
truncation logic:

kernel/bpf/verifier.c:check_tp_buffer_access() {
    ...
    env->prog->aux->max_tp_access = max(reg->var_off.value + off + size,
                                        env->prog->aux->max_tp_access);
    ...
}

Could this lead to out-of-bounds access at runtime?

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260604150755.2487555-1-gnq25@mails.tsinghua.edu.cn?part=1

[PATCH bpf 2/2] selftests/bpf: add tests for PTR_TO_FLOW_KEYS constant offset bounds

[Nuiqi Gui]

Add verifier tests covering constant pointer arithmetic on a
PTR_TO_FLOW_KEYS register, which regressed with commit 022ac0750883
("bpf: use reg->var_off instead of reg->off for pointers"): an
out-of-bounds offset introduced as flow_keys += K and then dereferenced
at insn->off 0 was accepted, while the equivalent flow_keys + K direct
offset was rejected.

The tests check that:
 - in-bounds constant arithmetic on the keys pointer is still accepted,
 - an out-of-bounds offset introduced via constant arithmetic is rejected
   for both read and write, with the same diagnostic as the direct
   insn->off form.

Signed-off-by: Nuiqi Gui <gnq25@mails.tsinghua.edu.cn>
---
 .../selftests/bpf/prog_tests/verifier.c       |  2 +
 .../selftests/bpf/progs/verifier_flow_keys.c  | 77 +++++++++++++++++++
 2 files changed, 79 insertions(+)
 create mode 100644 tools/testing/selftests/bpf/progs/verifier_flow_keys.c

diff --git a/tools/testing/selftests/bpf/prog_tests/verifier.c b/tools/testing/selftests/bpf/prog_tests/verifier.c
index 06cd24e37b3f..feaee223689b 100644
--- a/tools/testing/selftests/bpf/prog_tests/verifier.c
+++ b/tools/testing/selftests/bpf/prog_tests/verifier.c
@@ -38,6 +38,7 @@
 #include "verifier_div0.skel.h"
 #include "verifier_div_mod_bounds.skel.h"
 #include "verifier_div_overflow.skel.h"
+#include "verifier_flow_keys.skel.h"
 #include "verifier_global_subprogs.skel.h"
 #include "verifier_global_ptr_args.skel.h"
 #include "verifier_gotol.skel.h"
@@ -187,6 +188,7 @@ void test_verifier_direct_stack_access_wraparound(void) { RUN(verifier_direct_st
 void test_verifier_div0(void)                 { RUN(verifier_div0); }
 void test_verifier_div_mod_bounds(void)       { RUN(verifier_div_mod_bounds); }
 void test_verifier_div_overflow(void)         { RUN(verifier_div_overflow); }
+void test_verifier_flow_keys(void)            { RUN(verifier_flow_keys); }
 void test_verifier_global_subprogs(void)      { RUN(verifier_global_subprogs); }
 void test_verifier_global_ptr_args(void)      { RUN(verifier_global_ptr_args); }
 void test_verifier_gotol(void)                { RUN(verifier_gotol); }
diff --git a/tools/testing/selftests/bpf/progs/verifier_flow_keys.c b/tools/testing/selftests/bpf/progs/verifier_flow_keys.c
new file mode 100644
index 000000000000..512e5d1d2665
--- /dev/null
+++ b/tools/testing/selftests/bpf/progs/verifier_flow_keys.c
@@ -0,0 +1,77 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Constant-offset bounds checks for PTR_TO_FLOW_KEYS pointer arithmetic. */
+
+#include "vmlinux.h"
+#include <bpf/bpf_helpers.h>
+#include "bpf_misc.h"
+
+/* sizeof(struct bpf_flow_keys) is well under 4096, so +0x1000 is OOB. */
+
+SEC("flow_dissector")
+__description("flow_keys: in-bounds constant pointer arithmetic accepted")
+__success
+__naked void flow_keys_const_inbounds(void)
+{
+	asm volatile ("					\
+	r1 = *(u64 *)(r1 + %[flow_keys]);		\
+	r1 += 8;					\
+	r0 = *(u64 *)(r1 + 0);				\
+	r0 = 0;						\
+	exit;						\
+"	:
+	: __imm_const(flow_keys, offsetof(struct __sk_buff, flow_keys))
+	: __clobber_all);
+}
+
+SEC("flow_dissector")
+__description("flow_keys: OOB via constant pointer arithmetic rejected")
+__failure __msg("invalid access to flow keys off=4096 size=8")
+__naked void flow_keys_const_oob_read(void)
+{
+	asm volatile ("					\
+	r1 = *(u64 *)(r1 + %[flow_keys]);		\
+	r1 += 4096;					\
+	r0 = *(u64 *)(r1 + 0);				\
+	r0 = 0;						\
+	exit;						\
+"	:
+	: __imm_const(flow_keys, offsetof(struct __sk_buff, flow_keys))
+	: __clobber_all);
+}
+
+SEC("flow_dissector")
+__description("flow_keys: OOB write via constant pointer arithmetic rejected")
+__failure __msg("invalid access to flow keys off=4096 size=8")
+__naked void flow_keys_const_oob_write(void)
+{
+	asm volatile ("					\
+	r1 = *(u64 *)(r1 + %[flow_keys]);		\
+	r1 += 4096;					\
+	r2 = 0;						\
+	*(u64 *)(r1 + 0) = r2;				\
+	r0 = 0;						\
+	exit;						\
+"	:
+	: __imm_const(flow_keys, offsetof(struct __sk_buff, flow_keys))
+	: __clobber_all);
+}
+
+/* Equivalent OOB expressed directly in insn->off; this form was always
+ * rejected and is kept to show both forms now share one diagnostic.
+ */
+SEC("flow_dissector")
+__description("flow_keys: OOB via insn->off rejected")
+__failure __msg("invalid access to flow keys off=4096 size=8")
+__naked void flow_keys_insn_off_oob(void)
+{
+	asm volatile ("					\
+	r1 = *(u64 *)(r1 + %[flow_keys]);		\
+	r0 = *(u64 *)(r1 + 4096);			\
+	r0 = 0;						\
+	exit;						\
+"	:
+	: __imm_const(flow_keys, offsetof(struct __sk_buff, flow_keys))
+	: __clobber_all);
+}
+
+char _license[] SEC("license") = "GPL";
-- 
2.34.1

Re: [PATCH bpf 2/2] selftests/bpf: add tests for PTR_TO_FLOW_KEYS constant offset bounds

[sashiko-bot]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Low] BPF subsystem multi-line comment style violation.
--

commit 86bbbe949c13579e90a39499cabd9dc701da0bef
Author: Nuiqi Gui <gnq25@mails.tsinghua.edu.cn>

selftests/bpf: add tests for PTR_TO_FLOW_KEYS constant offset bounds

This commit adds verifier tests for constant pointer arithmetic on a
PTR_TO_FLOW_KEYS register to ensure bounds checking works correctly.
It verifies that in-bounds arithmetic is accepted and out-of-bounds
is properly rejected.

> diff --git a/tools/testing/selftests/bpf/progs/verifier_flow_keys.c b/tools/testing/selftests/bpf/progs/verifier_flow_keys.c
> new file mode 100644
> --- /dev/null
> +++ b/tools/testing/selftests/bpf/progs/verifier_flow_keys.c

[ ... ]

> +	: __imm_const(flow_keys, offsetof(struct __sk_buff, flow_keys))
> +	: __clobber_all);
> +}
> +
> +/* Equivalent OOB expressed directly in insn->off; this form was always
> + * rejected and is kept to show both forms now share one diagnostic.
> + */

[Severity: Low]
This isn't a bug, but does this multi-line comment follow the preferred
BPF subsystem style? The guidelines specify that the opening /* should be
on its own line, with the comment text starting on the next line.

> +SEC("flow_dissector")
> +__description("flow_keys: OOB via insn->off rejected")

[ ... ]

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260604150755.2487555-1-gnq25@mails.tsinghua.edu.cn?part=2

Re: [PATCH bpf 0/2] bpf, verifier: fix PTR_TO_FLOW_KEYS constant-offset OOB

[Alexei Starovoitov]

On Thu, Jun 4, 2026 at 8:08 AM Nuiqi Gui <gnq25@mails.tsinghua.edu.cn> wrote:
>
> A constant offset added to a PTR_TO_FLOW_KEYS register lands in
> reg->var_off, but check_flow_keys_access() bounds-checks only insn->off
> and never folds reg->var_off.value.  A BPF_PROG_TYPE_FLOW_DISSECTOR
> program can therefore do "flow_keys += 0x1000; *(flow_keys + 0)" and have
> it accepted, then read/write kernel stack past struct bpf_flow_keys at
> runtime.  Patch 1 folds reg->var_off.value into the offset (and rejects
> non-constant offsets), mirroring check_ctx_access(); patch 2 adds verifier
> selftests.
>
> This is a regression introduced in the 7.1 development cycle by commit
> 022ac0750883 ("bpf: use reg->var_off instead of reg->off for pointers"),
> which moved the constant offset from reg->off (folded generically before
> 022ac0750883) into reg->var_off without updating the flow_keys path.  No
> released kernel is affected: v7.0.x rejects the program above, and the bug
> reproduces only on v7.1-rc1..rc5, so no stable backport is needed.
>
> It was first reported privately to security@kernel.org; per their guidance
> it is handled in the open as a normal regression fix.

Right and the fix should target bpf-next.
It's not a security issue.

pw-bot: cr