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Date: Tue, 28 Jul 2020 10:23:17 -0700
From: Martin KaFai Lau <kafai@...com>
To: John Fastabend <john.fastabend@...il.com>
CC: <daniel@...earbox.net>, <ast@...nel.org>, <netdev@...r.kernel.org>,
<bpf@...r.kernel.org>
Subject: Re: [bpf PATCH 1/3] bpf: sock_ops ctx access may stomp registers in
corner case
On Tue, Jul 28, 2020 at 08:43:46AM -0700, 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.
Great debugging! Thanks for the details explanation.
>
> 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)
>
> 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")
I think this issue dated at least back from
commit 34d367c59233 ("bpf: Make SOCK_OPS_GET_TCP struct independent")
There are a few refactoring since then, so fixing in much older
code may not worth it since it is rare?
> Signed-off-by: John Fastabend <john.fastabend@...il.com>
> ---
> 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..c50cb80 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)); \
Instead of sock_ops->temp, can BPF_REG_AX be used here as a temp?
e.g. bpf_convert_shinfo_access() has already used it as a temp also.
Also, it seems the "sk" access in sock_ops_convert_ctx_access() suffers
a similar issue.
> + 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) \
>
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