The current x32 BPF JIT is incorrect for JMP32 JSET BPF_X when the upper 32 bits of operand registers are non-zero in certain situations.
The problem is in the following code:
case BPF_JMP | BPF_JSET | BPF_X: case BPF_JMP32 | BPF_JSET | BPF_X: ...
/* and dreg_lo,sreg_lo */ EMIT2(0x23, add_2reg(0xC0, sreg_lo, dreg_lo)); /* and dreg_hi,sreg_hi */ EMIT2(0x23, add_2reg(0xC0, sreg_hi, dreg_hi)); /* or dreg_lo,dreg_hi */ EMIT2(0x09, add_2reg(0xC0, dreg_lo, dreg_hi));
This code checks the upper bits of the operand registers regardless if the BPF instruction is BPF_JMP32 or BPF_JMP64. Registers dreg_hi and dreg_lo are not loaded from the stack for BPF_JMP32, however, they can still be polluted with values from previous instructions.
The following BPF program demonstrates the bug. The jset64 instruction loads the temporary registers and performs the jump, since ((u64)r7 & (u64)r8) is non-zero. The jset32 should _not_ be taken, as the lower 32 bits are all zero, however, the current JIT will take the branch due the pollution of temporary registers from the earlier jset64.
mov64 r0, 0 ld64 r7, 0x8000000000000000 ld64 r8, 0x8000000000000000 jset64 r7, r8, 1 exit jset32 r7, r8, 1 mov64 r0, 2 exit
The expected return value of this program is 2; under the buggy x32 JIT it returns 0. The fix is to skip using the upper 32 bits for jset32 and compare the upper 32 bits for jset64 only.
All tests in test_bpf.ko and selftests/bpf/test_verifier continue to pass with this change.
We found this bug using our automated verification tool, Serval.
Fixes: 69f827eb6e14 ("x32: bpf: implement jitting of JMP32") Co-developed-by: Xi Wang xi.wang@gmail.com Signed-off-by: Xi Wang xi.wang@gmail.com Signed-off-by: Luke Nelson luke.r.nels@gmail.com --- arch/x86/net/bpf_jit_comp32.c | 10 ++++++---- 1 file changed, 6 insertions(+), 4 deletions(-)
diff --git a/arch/x86/net/bpf_jit_comp32.c b/arch/x86/net/bpf_jit_comp32.c index 393d251798c0..4d2a7a764602 100644 --- a/arch/x86/net/bpf_jit_comp32.c +++ b/arch/x86/net/bpf_jit_comp32.c @@ -2039,10 +2039,12 @@ static int do_jit(struct bpf_prog *bpf_prog, int *addrs, u8 *image, } /* and dreg_lo,sreg_lo */ EMIT2(0x23, add_2reg(0xC0, sreg_lo, dreg_lo)); - /* and dreg_hi,sreg_hi */ - EMIT2(0x23, add_2reg(0xC0, sreg_hi, dreg_hi)); - /* or dreg_lo,dreg_hi */ - EMIT2(0x09, add_2reg(0xC0, dreg_lo, dreg_hi)); + if (is_jmp64) { + /* and dreg_hi,sreg_hi */ + EMIT2(0x23, add_2reg(0xC0, sreg_hi, dreg_hi)); + /* or dreg_lo,dreg_hi */ + EMIT2(0x09, add_2reg(0xC0, dreg_lo, dreg_hi)); + } goto emit_cond_jmp; } case BPF_JMP | BPF_JSET | BPF_K: