On 7/29/20 6:22 PM, John Fastabend wrote:
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,
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)
The diff below looks good to me, but I'm confused on this one above. I'm probably
missing something, but given this is the dst == src case with the r2 register, where
in the dump do we first saves the content of r9 into the scratch tmp store?
Line 19 seems to restore it, but the save is missing, no?
Please double check whether this was just omitted, but I would really like to have
the commit message 100% correct as it otherwise causes confusion when we stare at it
again a month later wrt what was the original intention.
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@xxxxxxxxx>
---
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 29e34551..15a0842 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -8314,15 +8314,31 @@ 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, \
@@ -8331,6 +8347,12 @@ 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) \
