[PATCH 5.7 068/124] bpf: sock_ops ctx access may stomp registers in corner case

Greg Kroah-Hartman
2020-08-24T08:43:14Z

From: John Fastabend <john.fastabend@gmail.com>

[ Upstream commit fd09af010788a884de1c39537c288830c3d305db ]

I had a sockmap program that after doing some refactoring started spewing
this splat at me:

[18610.807284] BUG: unable to handle kernel NULL pointer dereference at 0000000000000001
[...]
[18610.807359] Call Trace:
[18610.807370] ? 0xffffffffc114d0d5
[18610.807382] __cgroup_bpf_run_filter_sock_ops+0x7d/0xb0
[18610.807391] tcp_connect+0x895/0xd50
[18610.807400] tcp_v4_connect+0x465/0x4e0
[18610.807407] __inet_stream_connect+0xd6/0x3a0
[18610.807412] ? __inet_stream_connect+0x5/0x3a0
[18610.807417] inet_stream_connect+0x3b/0x60
[18610.807425] __sys_connect+0xed/0x120

After some debugging I was able to build this simple reproducer,

__section("sockops/reproducer_bad")
int bpf_reproducer_bad(struct bpf_sock_ops *skops)
{
volatile __maybe_unused __u32 i = skops->snd_ssthresh;
return 0;
}

And along the way noticed that below program ran without splat,

__section("sockops/reproducer_good")
int bpf_reproducer_good(struct bpf_sock_ops *skops)
{
volatile __maybe_unused __u32 i = skops->snd_ssthresh;
volatile __maybe_unused __u32 family;

compiler_barrier();

family = skops->family;
return 0;
}

So I decided to check out the code we generate for the above two
programs and noticed each generates the BPF code you would expect,

0000000000000000 <bpf_reproducer_bad>:
; volatile __maybe_unused __u32 i = skops->snd_ssthresh;
0: r1 = *(u32 *)(r1 + 96)
1: *(u32 *)(r10 - 4) = r1
; return 0;
2: r0 = 0
3: exit

0000000000000000 <bpf_reproducer_good>:
; volatile __maybe_unused __u32 i = skops->snd_ssthresh;
0: r2 = *(u32 *)(r1 + 96)
1: *(u32 *)(r10 - 4) = r2
; family = skops->family;
2: r1 = *(u32 *)(r1 + 20)
3: *(u32 *)(r10 - 8) = r1
; return 0;
4: r0 = 0
5: exit

So we get reasonable assembly, but still something was causing the null
pointer dereference. So, we load the programs and dump the xlated version
observing that line 0 above 'r* = *(u32 *)(r1 +96)' is going to be
translated by the skops access helpers.

int bpf_reproducer_bad(struct bpf_sock_ops * skops):
; volatile __maybe_unused __u32 i = skops->snd_ssthresh;
0: (61) r1 = *(u32 *)(r1 +28)
1: (15) if r1 == 0x0 goto pc+2
2: (79) r1 = *(u64 *)(r1 +0)
3: (61) r1 = *(u32 *)(r1 +2340)
; volatile __maybe_unused __u32 i = skops->snd_ssthresh;
4: (63) *(u32 *)(r10 -4) = r1
; return 0;
5: (b7) r0 = 0
6: (95) exit

int bpf_reproducer_good(struct bpf_sock_ops * skops):
; volatile __maybe_unused __u32 i = skops->snd_ssthresh;
0: (61) r2 = *(u32 *)(r1 +28)
1: (15) if r2 == 0x0 goto pc+2
2: (79) r2 = *(u64 *)(r1 +0)
3: (61) r2 = *(u32 *)(r2 +2340)
; volatile __maybe_unused __u32 i = skops->snd_ssthresh;
4: (63) *(u32 *)(r10 -4) = r2
; family = skops->family;
5: (79) r1 = *(u64 *)(r1 +0)
6: (69) r1 = *(u16 *)(r1 +16)
; family = skops->family;
7: (63) *(u32 *)(r10 -8) = r1
; return 0;
8: (b7) r0 = 0
9: (95) exit

Then we look at lines 0 and 2 above. In the good case we do the zero
check in r2 and then load 'r1 + 0' at line 2. Do a quick cross-check
into the bpf_sock_ops check and we can confirm that is the 'struct
sock *sk' pointer field. But, in the bad case,

0: (61) r1 = *(u32 *)(r1 +28)
1: (15) if r1 == 0x0 goto pc+2
2: (79) r1 = *(u64 *)(r1 +0)

Oh no, we read 'r1 +28' into r1, this is skops->fullsock and then in
line 2 we read the 'r1 +0' as a pointer. Now jumping back to our spat,

[18610.807284] BUG: unable to handle kernel NULL pointer dereference at 0000000000000001

The 0x01 makes sense because that is exactly the fullsock value. And
its not a valid dereference so we splat.

To fix we need to guard the case when a program is doing a sock_ops field
access with src_reg == dst_reg. This is already handled in the load case
where the ctx_access handler uses a tmp register being careful to
store the old value and restore it. To fix the get case test if
src_reg == dst_reg and in this case do the is_fullsock test in the
temporary register. Remembering to restore the temporary register before
writing to either dst_reg or src_reg to avoid smashing the pointer into
the struct holding the tmp variable.

Adding this inline code to test_tcpbpf_kern will now be generated
correctly from,

9: r2 = *(u32 *)(r2 + 96)

to xlated code,

12: (7b) *(u64 *)(r2 +32) = r9
13: (61) r9 = *(u32 *)(r2 +28)
14: (15) if r9 == 0x0 goto pc+4
15: (79) r9 = *(u64 *)(r2 +32)
16: (79) r2 = *(u64 *)(r2 +0)
17: (61) r2 = *(u32 *)(r2 +2348)
18: (05) goto pc+1
19: (79) r9 = *(u64 *)(r2 +32)

And in the normal case we keep the original code, because really this
is an edge case. From this,

9: r2 = *(u32 *)(r6 + 96)

to xlated code,

22: (61) r2 = *(u32 *)(r6 +28)
23: (15) if r2 == 0x0 goto pc+2
24: (79) r2 = *(u64 *)(r6 +0)
25: (61) r2 = *(u32 *)(r2 +2348)

So three additional instructions if dst == src register, but I scanned
my current code base and did not see this pattern anywhere so should
not be a big deal. Further, it seems no one else has hit this or at
least reported it so it must a fairly rare pattern.

Fixes: 9b1f3d6e5af29 ("bpf: Refactor sock_ops_convert_ctx_access")
Signed-off-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Song Liu <songliubraving@fb.com>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Link: https://lore.kernel.org/bpf/159718347772.4728.2781381670567919577.stgit@john-Precision-5820-Tower
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
net/core/filter.c | 26 ++++++++++++++++++++++++--
1 file changed, 24 insertions(+), 2 deletions(-)

diff --git a/net/core/filter.c b/net/core/filter.c
index cebbb6ba9ed92..dcc06e510ec5d 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -8066,15 +8066,31 @@ static u32 sock_ops_convert_ctx_access(enum bpf_access_type type,
/* Helper macro for adding read access to tcp_sock or sock fields. */
#define SOCK_OPS_GET_FIELD(BPF_FIELD, OBJ_FIELD, OBJ) \
do { \
+ int fullsock_reg = si->dst_reg, reg = BPF_REG_9, jmp = 2; \
BUILD_BUG_ON(sizeof_field(OBJ, OBJ_FIELD) > \
sizeof_field(struct bpf_sock_ops, BPF_FIELD)); \
+ if (si->dst_reg == reg || si->src_reg == reg) \
+ reg--; \
+ if (si->dst_reg == reg || si->src_reg == reg) \
+ reg--; \
+ if (si->dst_reg == si->src_reg) { \
+ *insn++ = BPF_STX_MEM(BPF_DW, si->src_reg, reg, \
+ offsetof(struct bpf_sock_ops_kern, \
+ temp)); \
+ fullsock_reg = reg; \
+ jmp += 2; \
+ } \
*insn++ = BPF_LDX_MEM(BPF_FIELD_SIZEOF( \
struct bpf_sock_ops_kern, \
is_fullsock), \
- si->dst_reg, si->src_reg, \
+ fullsock_reg, si->src_reg, \
offsetof(struct bpf_sock_ops_kern, \
is_fullsock)); \
- *insn++ = BPF_JMP_IMM(BPF_JEQ, si->dst_reg, 0, 2); \
+ *insn++ = BPF_JMP_IMM(BPF_JEQ, fullsock_reg, 0, jmp); \
+ if (si->dst_reg == si->src_reg) \
+ *insn++ = BPF_LDX_MEM(BPF_DW, reg, si->src_reg, \
+ offsetof(struct bpf_sock_ops_kern, \
+ temp)); \
*insn++ = BPF_LDX_MEM(BPF_FIELD_SIZEOF( \
struct bpf_sock_ops_kern, sk),\
si->dst_reg, si->src_reg, \
@@ -8083,6 +8099,12 @@ static u32 sock_ops_convert_ctx_access(enum bpf_access_type type,
OBJ_FIELD), \
si->dst_reg, si->dst_reg, \
offsetof(OBJ, OBJ_FIELD)); \
+ if (si->dst_reg == si->src_reg) { \
+ *insn++ = BPF_JMP_A(1); \
+ *insn++ = BPF_LDX_MEM(BPF_DW, reg, si->src_reg, \
+ offsetof(struct bpf_sock_ops_kern, \
+ temp)); \
+ } \
} while (0)

#define SOCK_OPS_GET_TCP_SOCK_FIELD(FIELD) \
--
2.25.1