[net] bpf, arm64: fix faulty emission of map access in tail calls

Submitted by Daniel Borkmann on May 10, 2017, 11:53 p.m.

Details

Message ID 0e928e7c351b962a16376a2ab29e2af1c5b4c027.1494459589.git.daniel@iogearbox.net
State Accepted
Delegated to: David Miller
Headers show

Commit Message

Daniel Borkmann May 10, 2017, 11:53 p.m.
Shubham was recently asking on netdev why in arm64 JIT we don't multiply
the index for accessing the tail call map by 8. That led me into testing
out arm64 JIT wrt tail calls and it turned out I got a NULL pointer
dereference on the tail call.

The buggy access is at:

  prog = array->ptrs[index];
  if (prog == NULL)
      goto out;

  [...]
  00000060:  d2800e0a  mov x10, #0x70 // #112
  00000064:  f86a682a  ldr x10, [x1,x10]
  00000068:  f862694b  ldr x11, [x10,x2]
  0000006c:  b40000ab  cbz x11, 0x00000080
  [...]

The code triggering the crash is f862694b. x1 at the time contains the
address of the bpf array, x10 offsetof(struct bpf_array, ptrs). Meaning,
above we load the pointer to the program at map slot 0 into x10. x10
can then be NULL if the slot is not occupied, which we later on try to
access with a user given offset in x2 that is the map index.

Fix this by emitting the following instead:

  [...]
  00000060:  d2800e0a  mov x10, #0x70 // #112
  00000064:  8b0a002a  add x10, x1, x10
  00000068:  d37df04b  lsl x11, x2, #3
  0000006c:  f86b694b  ldr x11, [x10,x11]
  00000070:  b40000ab  cbz x11, 0x00000084
  [...]

This basically adds the offset to ptrs to the base address of the bpf
array we got and we later on access the map with an index * 8 offset
relative to that. The tail call map itself is basically one large area
with meta data at the head followed by the array of prog pointers.
This makes tail calls working again, tested on Cavium ThunderX ARMv8.

Fixes: ddb55992b04d ("arm64: bpf: implement bpf_tail_call() helper")
Reported-by: Shubham Bansal <illusionist.neo@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
---
 arch/arm64/net/bpf_jit_comp.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

Comments

David Miller May 11, 2017, 4:42 p.m.
From: Daniel Borkmann <daniel@iogearbox.net>
Date: Thu, 11 May 2017 01:53:15 +0200

> Shubham was recently asking on netdev why in arm64 JIT we don't multiply
> the index for accessing the tail call map by 8. That led me into testing
> out arm64 JIT wrt tail calls and it turned out I got a NULL pointer
> dereference on the tail call.
 ...
> Fix this by emitting the following instead:
> 
>   [...]
>   00000060:  d2800e0a  mov x10, #0x70 // #112
>   00000064:  8b0a002a  add x10, x1, x10
>   00000068:  d37df04b  lsl x11, x2, #3
>   0000006c:  f86b694b  ldr x11, [x10,x11]
>   00000070:  b40000ab  cbz x11, 0x00000084
>   [...]
> 
> This basically adds the offset to ptrs to the base address of the bpf
> array we got and we later on access the map with an index * 8 offset
> relative to that. The tail call map itself is basically one large area
> with meta data at the head followed by the array of prog pointers.
> This makes tail calls working again, tested on Cavium ThunderX ARMv8.
> 
> Fixes: ddb55992b04d ("arm64: bpf: implement bpf_tail_call() helper")
> Reported-by: Shubham Bansal <illusionist.neo@gmail.com>
> Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

Applied and queued up for -stable, thanks.

Patch hide | download patch | download mbox

diff --git a/arch/arm64/net/bpf_jit_comp.c b/arch/arm64/net/bpf_jit_comp.c
index c6e5358..71f9305 100644
--- a/arch/arm64/net/bpf_jit_comp.c
+++ b/arch/arm64/net/bpf_jit_comp.c
@@ -253,8 +253,9 @@  static int emit_bpf_tail_call(struct jit_ctx *ctx)
 	 */
 	off = offsetof(struct bpf_array, ptrs);
 	emit_a64_mov_i64(tmp, off, ctx);
-	emit(A64_LDR64(tmp, r2, tmp), ctx);
-	emit(A64_LDR64(prg, tmp, r3), ctx);
+	emit(A64_ADD(1, tmp, r2, tmp), ctx);
+	emit(A64_LSL(1, prg, r3, 3), ctx);
+	emit(A64_LDR64(prg, tmp, prg), ctx);
 	emit(A64_CBZ(1, prg, jmp_offset), ctx);
 
 	/* goto *(prog->bpf_func + prologue_size); */