[PATCH v7 01/10] bpf/x86: fix JIT encoding of fixed-width immediates

Several places in the x86 JIT size an immediate with imm_size(), which
returns 1 or 4 bytes depending on the value. That is wrong for opcodes
whose immediate width is fixed by the encoding, and it breaks in both
directions.

TEST (0xF7 /0, used for BPF_JSET) has no imm8 form; the immediate is
always 32 bits. For a small mask such as BPF_JSET | BPF_K #0x1,
imm_size() returns 1, so the JIT emits a 1-byte immediate. The CPU
still consumes 4, swallowing 3 bytes of the following Jcc. The
instruction stream desyncs and the program crashes.

ROR and the shifts (0xC1 group) have the opposite problem: their
immediate is always imm8. For a count >= 128, imm_size() returns 4 and
the JIT emits 3 stray bytes, again desyncing the stream.

Size each immediate by its encoding: 32 bits for TEST, 8 bits for ROR
and the shifts.

Bugzilla ID: 1959
Fixes: cc752e43e079 ("bpf: add JIT compilation for x86_64 ISA")
Cc: [email protected]

Signed-off-by: Stephen Hemminger <[email protected]>
Acked-by: Marat Khalili <[email protected]>
Acked-by: Konstantin Ananyev <[email protected]>
---
 lib/bpf/bpf_jit_x86.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/lib/bpf/bpf_jit_x86.c b/lib/bpf/bpf_jit_x86.c
index 54eb279643..912d3f69bc 100644
--- a/lib/bpf/bpf_jit_x86.c
+++ b/lib/bpf/bpf_jit_x86.c
@@ -300,7 +300,7 @@ emit_ror_imm(struct bpf_jit_state *st, uint32_t dreg, uint32_t imm)
 	emit_rex(st, BPF_ALU, 0, dreg);
 	emit_bytes(st, &ops, sizeof(ops));
 	emit_modregrm(st, MOD_DIRECT, mods, dreg);
-	emit_imm(st, imm, imm_size(imm));
+	emit_imm(st, imm, sizeof(uint8_t));
 }
 
 /*
@@ -441,7 +441,7 @@ emit_shift_imm(struct bpf_jit_state *st, uint32_t op, uint32_t dreg,
 	uint32_t imm)
 {
 	emit_shift(st, op, dreg);
-	emit_imm(st, imm, imm_size(imm));
+	emit_imm(st, imm, sizeof(uint8_t));
 }
 
 /*
@@ -921,7 +921,7 @@ emit_tst_imm(struct bpf_jit_state *st, uint32_t op, uint32_t dreg, uint32_t imm)
 	emit_rex(st, op, 0, dreg);
 	emit_bytes(st, &ops, sizeof(ops));
 	emit_modregrm(st, MOD_DIRECT, mods, dreg);
-	emit_imm(st, imm, imm_size(imm));
+	emit_imm(st, imm, sizeof(int32_t));
 }
 
 static void
-- 
2.53.0