[PATCH bpf] bpf: Temporarily disable atomic operations in BPF arena

[PATCH bpf] bpf: Temporarily disable atomic operations in BPF arena

[Puranjay Mohan]

Currently, the x86 JIT handling PROBE_MEM32 tagged accesses is not
equipped to handle atomic accesses into PTR_TO_ARENA, as no PROBE_MEM32
tagging is performed and no handling is enabled for them.

This will lead to unsafety as the offset into arena will dereferenced
directly without turning it into a base + offset access into the arena
region.

Since the changes to the x86 JIT will be fairly involved, for now,
temporarily disallow use of PTR_TO_ARENA as the destination operand for
atomics until support is added to the JIT backend.

Fixes: 2fe99eb0ccf2 ("bpf: Add x86-64 JIT support for PROBE_MEM32 pseudo instructions.")
Reported-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Signed-off-by: Puranjay Mohan <puranjay12@gmail.com>
Acked-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
---
 kernel/bpf/verifier.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 63749ad5ac6b..1dd3b99d1bb9 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -5682,6 +5682,13 @@ static bool is_flow_key_reg(struct bpf_verifier_env *env, int regno)
 	return reg->type == PTR_TO_FLOW_KEYS;
 }
 
+static bool is_arena_reg(struct bpf_verifier_env *env, int regno)
+{
+	const struct bpf_reg_state *reg = reg_state(env, regno);
+
+	return reg->type == PTR_TO_ARENA;
+}
+
 static u32 *reg2btf_ids[__BPF_REG_TYPE_MAX] = {
 #ifdef CONFIG_NET
 	[PTR_TO_SOCKET] = &btf_sock_ids[BTF_SOCK_TYPE_SOCK],
@@ -7019,7 +7026,8 @@ static int check_atomic(struct bpf_verifier_env *env, int insn_idx, struct bpf_i
 	if (is_ctx_reg(env, insn->dst_reg) ||
 	    is_pkt_reg(env, insn->dst_reg) ||
 	    is_flow_key_reg(env, insn->dst_reg) ||
-	    is_sk_reg(env, insn->dst_reg)) {
+	    is_sk_reg(env, insn->dst_reg) ||
+	    is_arena_reg(env, insn->dst_reg)) {
 		verbose(env, "BPF_ATOMIC stores into R%d %s is not allowed\n",
 			insn->dst_reg,
 			reg_type_str(env, reg_state(env, insn->dst_reg)->type));
-- 
2.40.1

Re: [PATCH bpf] bpf: Temporarily disable atomic operations in BPF arena

[patchwork-bot+netdevbpf]

Hello:

This patch was applied to bpf/bpf.git (master)
by Alexei Starovoitov <ast@kernel.org>:

On Thu, 14 Mar 2024 17:49:31 +0000 you wrote:
> Currently, the x86 JIT handling PROBE_MEM32 tagged accesses is not
> equipped to handle atomic accesses into PTR_TO_ARENA, as no PROBE_MEM32
> tagging is performed and no handling is enabled for them.
> 
> This will lead to unsafety as the offset into arena will dereferenced
> directly without turning it into a base + offset access into the arena
> region.
> 
> [...]

Here is the summary with links:
  - [bpf] bpf: Temporarily disable atomic operations in BPF arena
    https://git.kernel.org/bpf/bpf/c/44d79142ede8

You are awesome, thank you!
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