From 1afebfeaf1eb31eadf28ec6902ef6eda548c2cc7 Mon Sep 17 00:00:00 2001 From: Daniel Sneddon Date: Mon, 5 May 2025 14:35:12 -0700 Subject: [PATCH] x86/bpf: Call branch history clearing sequence on exit commit d4e89d212d401672e9cdfe825d947ee3a9fbe3f5 upstream. Classic BPF programs have been identified as potential vectors for intra-mode Branch Target Injection (BTI) attacks. Classic BPF programs can be run by unprivileged users. They allow unprivileged code to execute inside the kernel. Attackers can use unprivileged cBPF to craft branch history in kernel mode that can influence the target of indirect branches. Introduce a branch history buffer (BHB) clearing sequence during the JIT compilation of classic BPF programs. The clearing sequence is the same as is used in previous mitigations to protect syscalls. Since eBPF programs already have their own mitigations in place, only insert the call on classic programs that aren't run by privileged users. Signed-off-by: Daniel Sneddon Signed-off-by: Pawan Gupta Signed-off-by: Dave Hansen Acked-by: Daniel Borkmann Reviewed-by: Alexandre Chartre Signed-off-by: Greg Kroah-Hartman --- arch/x86/net/bpf_jit_comp.c | 33 +++++++++++++++++++++++++++++++++ 1 file changed, 33 insertions(+) diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c index a50c99e9b5c0..4cdc301952b9 100644 --- a/arch/x86/net/bpf_jit_comp.c +++ b/arch/x86/net/bpf_jit_comp.c @@ -1072,6 +1072,30 @@ static void emit_shiftx(u8 **pprog, u32 dst_reg, u8 src_reg, bool is64, u8 op) #define RESTORE_TAIL_CALL_CNT(stack) \ EMIT3_off32(0x48, 0x8B, 0x85, -round_up(stack, 8) - 8) +static int emit_spectre_bhb_barrier(u8 **pprog, u8 *ip, + struct bpf_prog *bpf_prog) +{ + u8 *prog = *pprog; + u8 *func; + + if (cpu_feature_enabled(X86_FEATURE_CLEAR_BHB_LOOP)) { + /* The clearing sequence clobbers eax and ecx. */ + EMIT1(0x50); /* push rax */ + EMIT1(0x51); /* push rcx */ + ip += 2; + + func = (u8 *)clear_bhb_loop; + ip += x86_call_depth_emit_accounting(&prog, func); + + if (emit_call(&prog, func, ip)) + return -EINVAL; + EMIT1(0x59); /* pop rcx */ + EMIT1(0x58); /* pop rax */ + } + *pprog = prog; + return 0; +} + static int do_jit(struct bpf_prog *bpf_prog, int *addrs, u8 *image, u8 *rw_image, int oldproglen, struct jit_context *ctx, bool jmp_padding) { @@ -1945,6 +1969,15 @@ emit_jmp: seen_exit = true; /* Update cleanup_addr */ ctx->cleanup_addr = proglen; + + if (bpf_prog_was_classic(bpf_prog) && + !capable(CAP_SYS_ADMIN)) { + u8 *ip = image + addrs[i - 1]; + + if (emit_spectre_bhb_barrier(&prog, ip, bpf_prog)) + return -EINVAL; + } + pop_callee_regs(&prog, callee_regs_used); EMIT1(0xC9); /* leave */ emit_return(&prog, image + addrs[i - 1] + (prog - temp));