[bpf-next,v2,13/15] selftests/bpf: Add test cases for narrowing fill

Commit Message

Maxim Mikityanskiy Jan. 8, 2024, 8:52 p.m. UTC

From: Maxim Mikityanskiy <[e-mail]>

The previous commit allowed to preserve boundaries and track IDs of
scalars on narrowing fills. Add test cases for that pattern.

Signed-off-by: Maxim Mikityanskiy <[e-mail]>
Acked-by: Eduard Zingerman <[e-mail]>
---
 .../selftests/bpf/progs/verifier_spill_fill.c | 108 ++++++++++++++++++
 1 file changed, 108 insertions(+)

Comments

Andrii Nakryiko Jan. 9, 2024, 11:55 p.m. UTC | #1

On Mon, Jan 8, 2024 at 12:53 PM Maxim Mikityanskiy <[e-mail]> wrote:
>
> From: Maxim Mikityanskiy <[e-mail]>
>
> The previous commit allowed to preserve boundaries and track IDs of
> scalars on narrowing fills. Add test cases for that pattern.
>
> Signed-off-by: Maxim Mikityanskiy <[e-mail]>
> Acked-by: Eduard Zingerman <[e-mail]>
> ---
>  .../selftests/bpf/progs/verifier_spill_fill.c | 108 ++++++++++++++++++
>  1 file changed, 108 insertions(+)
>
> diff --git a/tools/testing/selftests/bpf/progs/verifier_spill_fill.c b/tools/testing/selftests/bpf/progs/verifier_spill_fill.c
> index fab8ae9fe947..3764111d190d 100644
> --- a/tools/testing/selftests/bpf/progs/verifier_spill_fill.c
> +++ b/tools/testing/selftests/bpf/progs/verifier_spill_fill.c
> @@ -936,4 +936,112 @@ l0_%=:    r0 = 0;                                         \
>         : __clobber_all);
>  }
>
> +SEC("xdp")
> +__description("32-bit fill after 64-bit spill")
> +__success __retval(0)
> +__naked void fill_32bit_after_spill_64bit(void)

I guess these tests are an answer for my question about mixing
spill/fill sizes on earlier patch (so disregard those)

> +{
> +       asm volatile("                                  \
> +       /* Randomize the upper 32 bits. */              \
> +       call %[bpf_get_prandom_u32];                    \
> +       r0 <<= 32;                                      \
> +       /* 64-bit spill r0 to stack. */                 \
> +       *(u64*)(r10 - 8) = r0;                          \
> +       /* 32-bit fill r0 from stack. */                \
> +       r0 = *(u32*)(r10 - %[offset]);                          \

have you considered doing the BYTE_ORDER check right here and have
offset embedded in assembly instruction directly:

#if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
    r0 = *(u32*)(r10 - 8);
#else
    r0 = *(u32*)(r10 - 4);
#endif

It's a bit less jumping around the code when reading. And it's kind of
obviously that this is endianness-dependent without jumping to
definition of %[offset]?


> +       /* Boundary check on r0 with predetermined result. */\
> +       if r0 == 0 goto l0_%=;                          \
> +       /* Dead branch: the verifier should prune it. Do an invalid memory\
> +        * access if the verifier follows it.           \
> +        */                                             \
> +       r0 = *(u64*)(r9 + 0);                           \
> +l0_%=: exit;                                           \
> +"      :
> +       : __imm(bpf_get_prandom_u32),
> +#if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
> +         __imm_const(offset, 8)
> +#else
> +         __imm_const(offset, 4)
> +#endif
> +       : __clobber_all);
> +}
> +

[...]

Patch

diff --git a/tools/testing/selftests/bpf/progs/verifier_spill_fill.c b/tools/testing/selftests/bpf/progs/verifier_spill_fill.c
index fab8ae9fe947..3764111d190d 100644
--- a/tools/testing/selftests/bpf/progs/verifier_spill_fill.c
+++ b/tools/testing/selftests/bpf/progs/verifier_spill_fill.c
@@ -936,4 +936,112 @@  l0_%=:	r0 = 0;						\
 	: __clobber_all);
 }
 
+SEC("xdp")
+__description("32-bit fill after 64-bit spill")
+__success __retval(0)
+__naked void fill_32bit_after_spill_64bit(void)
+{
+	asm volatile("					\
+	/* Randomize the upper 32 bits. */		\
+	call %[bpf_get_prandom_u32];			\
+	r0 <<= 32;					\
+	/* 64-bit spill r0 to stack. */			\
+	*(u64*)(r10 - 8) = r0;				\
+	/* 32-bit fill r0 from stack. */		\
+	r0 = *(u32*)(r10 - %[offset]);				\
+	/* Boundary check on r0 with predetermined result. */\
+	if r0 == 0 goto l0_%=;				\
+	/* Dead branch: the verifier should prune it. Do an invalid memory\
+	 * access if the verifier follows it.		\
+	 */						\
+	r0 = *(u64*)(r9 + 0);				\
+l0_%=:	exit;						\
+"	:
+	: __imm(bpf_get_prandom_u32),
+#if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
+	  __imm_const(offset, 8)
+#else
+	  __imm_const(offset, 4)
+#endif
+	: __clobber_all);
+}
+
+SEC("xdp")
+__description("32-bit fill after 64-bit spill of 32-bit value should preserve ID")
+__success __retval(0)
+__naked void fill_32bit_after_spill_64bit_preserve_id(void)
+{
+	asm volatile ("					\
+	/* Randomize the lower 32 bits. */		\
+	call %[bpf_get_prandom_u32];			\
+	w0 &= 0xffffffff;				\
+	/* 64-bit spill r0 to stack - should assign an ID. */\
+	*(u64*)(r10 - 8) = r0;				\
+	/* 32-bit fill r1 from stack - should preserve the ID. */\
+	r1 = *(u32*)(r10 - %[offset]);				\
+	/* Compare r1 with another register to trigger find_equal_scalars. */\
+	r2 = 0;						\
+	if r1 != r2 goto l0_%=;				\
+	/* The result of this comparison is predefined. */\
+	if r0 == r2 goto l0_%=;				\
+	/* Dead branch: the verifier should prune it. Do an invalid memory\
+	 * access if the verifier follows it.		\
+	 */						\
+	r0 = *(u64*)(r9 + 0);				\
+	exit;						\
+l0_%=:	r0 = 0;						\
+	exit;						\
+"	:
+	: __imm(bpf_get_prandom_u32),
+#if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
+	  __imm_const(offset, 8)
+#else
+	  __imm_const(offset, 4)
+#endif
+	: __clobber_all);
+}
+
+SEC("xdp")
+__description("32-bit fill after 64-bit spill should clear ID")
+__failure __msg("math between ctx pointer and 4294967295 is not allowed")
+__naked void fill_32bit_after_spill_64bit_clear_id(void)
+{
+	asm volatile ("					\
+	r6 = r1;					\
+	/* Roll one bit to force the verifier to track both branches. */\
+	call %[bpf_get_prandom_u32];			\
+	r0 &= 0x8;					\
+	/* Put a large number into r1. */		\
+	r1 = 0xffffffff;				\
+	r1 <<= 32;					\
+	r1 += r0;					\
+	/* 64-bit spill r1 to stack - should assign an ID. */\
+	*(u64*)(r10 - 8) = r1;				\
+	/* 32-bit fill r2 from stack - should clear the ID. */\
+	r2 = *(u32*)(r10 - %[offset]);			\
+	/* Compare r2 with another register to trigger find_equal_scalars.\
+	 * Having one random bit is important here, otherwise the verifier cuts\
+	 * the corners. If the ID was mistakenly preserved on fill, this would\
+	 * cause the verifier to think that r1 is also equal to zero in one of\
+	 * the branches, and equal to eight on the other branch.\
+	 */						\
+	r3 = 0;						\
+	if r2 != r3 goto l0_%=;				\
+l0_%=:	r1 >>= 32;					\
+	/* The verifier shouldn't propagate r2's range to r1, so it should\
+	 * still remember r1 = 0xffffffff and reject the below.\
+	 */						\
+	r6 += r1;					\
+	r0 = *(u32*)(r6 + 0);				\
+	exit;						\
+"	:
+	: __imm(bpf_get_prandom_u32),
+#if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
+	  __imm_const(offset, 8)
+#else
+	  __imm_const(offset, 4)
+#endif
+	: __clobber_all);
+}
+
 char _license[] SEC("license") = "GPL";