[PATCH 4.14 1/4] bpf: Do not use ax register in interpreter on div/mod
From: Thadeu Lima de Souza Cascardo <hidden>
Date: 2021-08-30 18:32:38
Also in:
stable
Subsystem:
bpf [core], bpf [general] (safe dynamic programs and tools), the rest · Maintainers:
Alexei Starovoitov, Daniel Borkmann, Andrii Nakryiko, Eduard Zingerman, Kumar Kartikeya Dwivedi, Linus Torvalds
From: Daniel Borkmann <daniel@iogearbox.net>
Partially undo old commit 144cd91c4c2b ("bpf: move tmp variable into ax
register in interpreter"). The reason we need this here is because ax
register will be used for holding temporary state for div/mod instruction
which otherwise interpreter would corrupt. This will cause a small +8 byte
stack increase for interpreter, but with the gain that we can use it from
verifier rewrites as scratch register.
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: John Fastabend <john.fastabend@gmail.com>
[cascardo: This partial revert is needed in order to support using AX for
the following two commits, as there is no JMP32 on 4.19.y]
Signed-off-by: Thadeu Lima de Souza Cascardo <redacted>
---
kernel/bpf/core.c | 32 +++++++++++++++-----------------
1 file changed, 15 insertions(+), 17 deletions(-)
diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
index e7211b0fa27c..30d871be9974 100644
--- a/kernel/bpf/core.c
+++ b/kernel/bpf/core.c@@ -616,9 +616,6 @@ static int bpf_jit_blind_insn(const struct bpf_insn *from, * below. * * Constant blinding is only used by JITs, not in the interpreter. - * The interpreter uses AX in some occasions as a local temporary - * register e.g. in DIV or MOD instructions. - * * In restricted circumstances, the verifier can also use the AX * register for rewrites as long as they do not interfere with * the above cases!
@@ -951,6 +948,7 @@ static unsigned int ___bpf_prog_run(u64 *regs, const struct bpf_insn *insn, u32 tail_call_cnt = 0; void *ptr; int off; + u64 tmp; #define CONT ({ insn++; goto select_insn; }) #define CONT_JMP ({ insn++; goto select_insn; })
@@ -1013,22 +1011,22 @@ static unsigned int ___bpf_prog_run(u64 *regs, const struct bpf_insn *insn, ALU64_MOD_X: if (unlikely(SRC == 0)) return 0; - div64_u64_rem(DST, SRC, &AX); - DST = AX; + div64_u64_rem(DST, SRC, &tmp); + DST = tmp; CONT; ALU_MOD_X: if (unlikely((u32)SRC == 0)) return 0; - AX = (u32) DST; - DST = do_div(AX, (u32) SRC); + tmp = (u32) DST; + DST = do_div(tmp, (u32) SRC); CONT; ALU64_MOD_K: - div64_u64_rem(DST, IMM, &AX); - DST = AX; + div64_u64_rem(DST, IMM, &tmp); + DST = tmp; CONT; ALU_MOD_K: - AX = (u32) DST; - DST = do_div(AX, (u32) IMM); + tmp = (u32) DST; + DST = do_div(tmp, (u32) IMM); CONT; ALU64_DIV_X: if (unlikely(SRC == 0))
@@ -1038,17 +1036,17 @@ static unsigned int ___bpf_prog_run(u64 *regs, const struct bpf_insn *insn, ALU_DIV_X: if (unlikely((u32)SRC == 0)) return 0; - AX = (u32) DST; - do_div(AX, (u32) SRC); - DST = (u32) AX; + tmp = (u32) DST; + do_div(tmp, (u32) SRC); + DST = (u32) tmp; CONT; ALU64_DIV_K: DST = div64_u64(DST, IMM); CONT; ALU_DIV_K: - AX = (u32) DST; - do_div(AX, (u32) IMM); - DST = (u32) AX; + tmp = (u32) DST; + do_div(tmp, (u32) IMM); + DST = (u32) tmp; CONT; ALU_END_TO_BE: switch (IMM) {
--
2.30.2