[PATCH bpf-next v5 0/3] bpf: fix verification of indirect var-off stack access

[PATCH bpf-next v5 0/3] bpf: fix verification of indirect var-off stack access

[Andrei Matei]

V4 to V5:
  - split the test into a separate patch

V3 to V4:
  - include a test per Eduard's request
  - target bpf-next per Alexei's request (patches didn't change)


V2 to V3:
  - simplify checks for max_off (don't call
    check_stack_slot_within_bounds for it)
  - append a commit to protect against overflow in the addition of the
    register and the offset

V1 to V2:
  - fix max_off calculation for access size = 0


Andrei Matei (3):
  bpf: fix verification of indirect var-off stack access
  bpf: add verifier regression test for previous patch
  bpf: guard stack limits against 32bit overflow

 kernel/bpf/verifier.c                         | 20 +++++--------
 .../selftests/bpf/progs/verifier_var_off.c    | 29 +++++++++++++++++++
 2 files changed, 36 insertions(+), 13 deletions(-)

-- 
2.40.1

[PATCH bpf-next v5 1/3] bpf: fix verification of indirect var-off stack access

[Andrei Matei]

This patch fixes a bug around the verification of possibly-zero-sized
stack accesses. When the access was done through a var-offset stack
pointer, check_stack_access_within_bounds was incorrectly computing the
maximum-offset of a zero-sized read to be the same as the register's min
offset. Instead, we have to take in account the register's maximum
possible value. The patch also simplifies how the max offset is checked;
the check is now simpler than for min offset.

The bug was allowing accesses to erroneously pass the
check_stack_access_within_bounds() checks, only to later crash in
check_stack_range_initialized() when all the possibly-affected stack
slots are iterated (this time with a correct max offset).
check_stack_range_initialized() is relying on
check_stack_access_within_bounds() for its accesses to the
stack-tracking vector to be within bounds; in the case of zero-sized
accesses, we were essentially only verifying that the lowest possible
slot was within bounds. We would crash when the max-offset of the stack
pointer was >= 0 (which shouldn't pass verification, and hopefully is
not something anyone's code attempts to do in practice).

Thanks Hao for reporting!

Reported-by: Hao Sun <sunhao.th@gmail.com>
Fixes: 01f810ace9ed3 ("bpf: Allow variable-offset stack access")
Closes: https://lore.kernel.org/bpf/CACkBjsZGEUaRCHsmaX=h-efVogsRfK1FPxmkgb0Os_frnHiNdw@mail.gmail.com/
Signed-off-by: Andrei Matei <andreimatei1@gmail.com>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
---
 kernel/bpf/verifier.c | 14 ++++----------
 1 file changed, 4 insertions(+), 10 deletions(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index e5ce530641ba..137240681fa9 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -6620,10 +6620,7 @@ static int check_stack_access_within_bounds(
 
 	if (tnum_is_const(reg->var_off)) {
 		min_off = reg->var_off.value + off;
-		if (access_size > 0)
-			max_off = min_off + access_size - 1;
-		else
-			max_off = min_off;
+		max_off = min_off + access_size;
 	} else {
 		if (reg->smax_value >= BPF_MAX_VAR_OFF ||
 		    reg->smin_value <= -BPF_MAX_VAR_OFF) {
@@ -6632,15 +6629,12 @@ static int check_stack_access_within_bounds(
 			return -EACCES;
 		}
 		min_off = reg->smin_value + off;
-		if (access_size > 0)
-			max_off = reg->smax_value + off + access_size - 1;
-		else
-			max_off = min_off;
+		max_off = reg->smax_value + off + access_size;
 	}
 
 	err = check_stack_slot_within_bounds(min_off, state, type);
-	if (!err)
-		err = check_stack_slot_within_bounds(max_off, state, type);
+	if (!err && max_off > 0)
+		err = -EINVAL; /* out of stack access into non-negative offsets */
 
 	if (err) {
 		if (tnum_is_const(reg->var_off)) {
-- 
2.40.1

[PATCH bpf-next v5 2/3] bpf: add verifier regression test for previous patch

[Andrei Matei]

Add a regression test for var-off zero-sized reads.

Acked-by: Eduard Zingerman <eddyz87@gmail.com>
---
 .../selftests/bpf/progs/verifier_var_off.c    | 29 +++++++++++++++++++
 1 file changed, 29 insertions(+)

diff --git a/tools/testing/selftests/bpf/progs/verifier_var_off.c b/tools/testing/selftests/bpf/progs/verifier_var_off.c
index 83a90afba785..b7bdd7db3a35 100644
--- a/tools/testing/selftests/bpf/progs/verifier_var_off.c
+++ b/tools/testing/selftests/bpf/progs/verifier_var_off.c
@@ -224,6 +224,35 @@ __naked void access_max_out_of_bound(void)
 	: __clobber_all);
 }
 
+/* Similar to the test above, but this time check the special case of a
+ * zero-sized stack access. We used to have a bug causing crashes for zero-sized
+ * out-of-bounds accesses.
+ */
+SEC("socket")
+__description("indirect variable-offset stack access, zero-sized, max out of bound")
+__failure __msg("invalid variable-offset indirect access to stack R1")
+__naked void zero_sized_access_max_out_of_bound(void)
+{
+	asm volatile ("                      \
+	r0 = 0;                              \
+	/* Fill some stack */                \
+	*(u64*)(r10 - 16) = r0;              \
+	*(u64*)(r10 - 8) = r0;               \
+	/* Get an unknown value */           \
+	r1 = *(u32*)(r1 + 0);                \
+	r1 &= 63;                            \
+	r1 += -16;                           \
+	/* r1 is now anywhere in [-16,48) */ \
+	r1 += r10;                           \
+	r2 = 0;                              \
+	r3 = 0;                              \
+	call %[bpf_probe_read_kernel];       \
+	exit;                                \
+"	:
+	: __imm(bpf_probe_read_kernel)
+	: __clobber_all);
+}
+
 SEC("lwt_in")
 __description("indirect variable-offset stack access, min out of bound")
 __failure __msg("invalid variable-offset indirect access to stack R2")
-- 
2.40.1

[PATCH bpf-next v5 3/3] bpf: guard stack limits against 32bit overflow

[Andrei Matei]

This patch promotes the arithmetic around checking stack bounds to be
done in the 64-bit domain, instead of the current 32bit. The arithmetic
implies adding together a 64-bit register with a int offset. The
register was checked to be below 1<<29 when it was variable, but not
when it was fixed. The offset either comes from an instruction (in which
case it is 16 bit), from another register (in which case the caller
checked it to be below 1<<29 [1]), or from the size of an argument to a
kfunc (in which case it can be a u32 [2]). Between the register being
inconsistently checked to be below 1<<29, and the offset being up to an
u32, it appears that we were open to overflowing the `int`s which were
currently used for arithmetic.

[1] https://github.com/torvalds/linux/blob/815fb87b753055df2d9e50f6cd80eb10235fe3e9/kernel/bpf/verifier.c#L7494-L7498
[2] https://github.com/torvalds/linux/blob/815fb87b753055df2d9e50f6cd80eb10235fe3e9/kernel/bpf/verifier.c#L11904

Signed-off-by: Andrei Matei <andreimatei1@gmail.com>
Reported-by: Andrii Nakryiko <andrii.nakryiko@gmail.com>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
---
 kernel/bpf/verifier.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 137240681fa9..6832ed743765 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -6577,7 +6577,7 @@ static int check_ptr_to_map_access(struct bpf_verifier_env *env,
  * The minimum valid offset is -MAX_BPF_STACK for writes, and
  * -state->allocated_stack for reads.
  */
-static int check_stack_slot_within_bounds(int off,
+static int check_stack_slot_within_bounds(s64 off,
 					  struct bpf_func_state *state,
 					  enum bpf_access_type t)
 {
@@ -6606,7 +6606,7 @@ static int check_stack_access_within_bounds(
 	struct bpf_reg_state *regs = cur_regs(env);
 	struct bpf_reg_state *reg = regs + regno;
 	struct bpf_func_state *state = func(env, reg);
-	int min_off, max_off;
+	s64 min_off, max_off;
 	int err;
 	char *err_extra;
 
@@ -6619,7 +6619,7 @@ static int check_stack_access_within_bounds(
 		err_extra = " write to";
 
 	if (tnum_is_const(reg->var_off)) {
-		min_off = reg->var_off.value + off;
+		min_off = (s64)reg->var_off.value + off;
 		max_off = min_off + access_size;
 	} else {
 		if (reg->smax_value >= BPF_MAX_VAR_OFF ||
-- 
2.40.1

Re: [PATCH bpf-next v5 0/3] bpf: fix verification of indirect var-off stack access

[patchwork-bot+netdevbpf]

Hello:

This series was applied to bpf/bpf-next.git (master)
by Andrii Nakryiko <andrii@kernel.org>:

On Wed,  6 Dec 2023 23:11:47 -0500 you wrote:
> V4 to V5:
>   - split the test into a separate patch
> 
> V3 to V4:
>   - include a test per Eduard's request
>   - target bpf-next per Alexei's request (patches didn't change)
> 
> [...]

Here is the summary with links:
  - [bpf-next,v5,1/3] bpf: fix verification of indirect var-off stack access
    https://git.kernel.org/bpf/bpf-next/c/4f114bc280bb
  - [bpf-next,v5,2/3] bpf: add verifier regression test for previous patch
    https://git.kernel.org/bpf/bpf-next/c/ec32ca301faa
  - [bpf-next,v5,3/3] bpf: guard stack limits against 32bit overflow
    https://git.kernel.org/bpf/bpf-next/c/755f82668d81

You are awesome, thank you!
-- 
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