[PATCH 5.14 118/172] bpf, x86: Fix bpf mapping of atomic fetch implementation

Greg Kroah-Hartman
2021-10-04T13:37:25Z

From: Johan Almbladh <johan.almbladh@anyfinetworks.com>

[ Upstream commit ced185824c89b60e65b5a2606954c098320cdfb8 ]

Fix the case where the dst register maps to %rax as otherwise this produces
an incorrect mapping with the implementation in 981f94c3e921 ("bpf: Add
bitwise atomic instructions") as %rax is clobbered given it's part of the
cmpxchg as operand.

The issue is similar to b29dd96b905f ("bpf, x86: Fix BPF_FETCH atomic and/or/
xor with r0 as src") just that the case of dst register was missed.

Before, dst=r0 (%rax) src=r2 (%rsi):

[...]
c5: mov %rax,%r10
c8: mov 0x0(%rax),%rax <---+ (broken)
cc: mov %rax,%r11 |
cf: and %rsi,%r11 |
d2: lock cmpxchg %r11,0x0(%rax) <---+
d8: jne 0x00000000000000c8 |
da: mov %rax,%rsi |
dd: mov %r10,%rax |
[...] |
|
After, dst=r0 (%rax) src=r2 (%rsi): |
|
[...] |
da: mov %rax,%r10 |
dd: mov 0x0(%r10),%rax <---+ (fixed)
e1: mov %rax,%r11 |
e4: and %rsi,%r11 |
e7: lock cmpxchg %r11,0x0(%r10) <---+
ed: jne 0x00000000000000dd
ef: mov %rax,%rsi
f2: mov %r10,%rax
[...]

The remaining combinations were fine as-is though:

After, dst=r9 (%r15) src=r0 (%rax):

[...]
dc: mov %rax,%r10
df: mov 0x0(%r15),%rax
e3: mov %rax,%r11
e6: and %r10,%r11
e9: lock cmpxchg %r11,0x0(%r15)
ef: jne 0x00000000000000df _
f1: mov %rax,%r10 | (unneeded, but
f4: mov %r10,%rax _| not a problem)
[...]

After, dst=r9 (%r15) src=r4 (%rcx):

[...]
de: mov %rax,%r10
e1: mov 0x0(%r15),%rax
e5: mov %rax,%r11
e8: and %rcx,%r11
eb: lock cmpxchg %r11,0x0(%r15)
f1: jne 0x00000000000000e1
f3: mov %rax,%rcx
f6: mov %r10,%rax
[...]

The case of dst == src register is rejected by the verifier and
therefore not supported, but x86 JIT also handles this case just
fine.

After, dst=r0 (%rax) src=r0 (%rax):

[...]
eb: mov %rax,%r10
ee: mov 0x0(%r10),%rax
f2: mov %rax,%r11
f5: and %r10,%r11
f8: lock cmpxchg %r11,0x0(%r10)
fe: jne 0x00000000000000ee
100: mov %rax,%r10
103: mov %r10,%rax
[...]

Fixes: 981f94c3e921 ("bpf: Add bitwise atomic instructions")
Reported-by: Johan Almbladh <johan.almbladh@anyfinetworks.com>
Signed-off-by: Johan Almbladh <johan.almbladh@anyfinetworks.com>
Co-developed-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Brendan Jackman <jackmanb@google.com>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
arch/x86/net/bpf_jit_comp.c | 13 ++++++++-----
1 file changed, 8 insertions(+), 5 deletions(-)

diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
index 47780844598a..ffcc4d29ad50 100644
--- a/arch/x86/net/bpf_jit_comp.c
+++ b/arch/x86/net/bpf_jit_comp.c
@@ -1341,9 +1341,10 @@ st: if (is_imm8(insn->off))
if (insn->imm == (BPF_AND | BPF_FETCH) ||
insn->imm == (BPF_OR | BPF_FETCH) ||
insn->imm == (BPF_XOR | BPF_FETCH)) {
- u8 *branch_target;
bool is64 = BPF_SIZE(insn->code) == BPF_DW;
u32 real_src_reg = src_reg;
+ u32 real_dst_reg = dst_reg;
+ u8 *branch_target;

/*
* Can't be implemented with a single x86 insn.
@@ -1354,11 +1355,13 @@ st: if (is_imm8(insn->off))
emit_mov_reg(&prog, true, BPF_REG_AX, BPF_REG_0);
if (src_reg == BPF_REG_0)
real_src_reg = BPF_REG_AX;
+ if (dst_reg == BPF_REG_0)
+ real_dst_reg = BPF_REG_AX;

branch_target = prog;
/* Load old value */
emit_ldx(&prog, BPF_SIZE(insn->code),
- BPF_REG_0, dst_reg, insn->off);
+ BPF_REG_0, real_dst_reg, insn->off);
/*
* Perform the (commutative) operation locally,
* put the result in the AUX_REG.
@@ -1369,7 +1372,8 @@ st: if (is_imm8(insn->off))
add_2reg(0xC0, AUX_REG, real_src_reg));
/* Attempt to swap in new value */
err = emit_atomic(&prog, BPF_CMPXCHG,
- dst_reg, AUX_REG, insn->off,
+ real_dst_reg, AUX_REG,
+ insn->off,
BPF_SIZE(insn->code));
if (WARN_ON(err))
return err;
@@ -1383,11 +1387,10 @@ st: if (is_imm8(insn->off))
/* Restore R0 after clobbering RAX */
emit_mov_reg(&prog, true, BPF_REG_0, BPF_REG_AX);
break;
-
}

err = emit_atomic(&prog, insn->imm, dst_reg, src_reg,
- insn->off, BPF_SIZE(insn->code));
+ insn->off, BPF_SIZE(insn->code));
if (err)
return err;
break;
--
2.33.0