bpf, x86: Clear fetch destination on faulting arena atomic

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populate_extable() encodes "there is no destination register to clear" as
DONT_CLEAR in the DST_REG field of the exception table metadata, and later
ex_handler_bpf() then reuses that very value to derive the direction it
reports the fault with is_write = (reg == DONT_CLEAR). The two coincide
for a plain load or store, but not for a RMW carrying BPF_FETCH. Such an
atomic writes memory, so it has to be reported as a WRITE, and it also reads
the old value into a register, src_reg for BPF_ADD | BPF_FETCH and BPF_XCHG,
r0 for BPF_CMPXCHG, so that register has to be cleared on fault. A single
DONT_CLEAR cannot say both, and the store branch picks it unconditionally:

  [...]
  } else {
          arena_reg = reg2pt_regs[dst_reg];
          fixup_reg = DONT_CLEAR;
  }
  [...]

The reported direction is therefore right, but on a fault over an unmapped
arena page the fetch destination keeps whatever it held before the atomic,
where every other BPF_PROBE_* access delivers 0. Give the metadata its own
ARENA_WRITE bit so that the reported direction no longer depends on whether
there is a register to clear, and fill DST_REG in from bpf_atomic_load_reg().
BPF_{AND,OR,XOR} | BPF_FETCH need no handling here, bpf_jit_supports_insn()
already rejects those in the arena.

Fixes: d503a04f8b ("bpf: Add support for certain atomics in bpf_arena to x86 JIT")
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Puranjay Mohan <puranjay@kernel.org>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://patch.msgid.link/20260811131600.506721-3-daniel@iogearbox.net
Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>

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Daniel Borkmann 2026-08-11 15:15:57 +02:00 • committed by Eduard Zingerman

parent 1519f488e8

commit cf92a10860

1 changed files with 27 additions and 8 deletions

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arch/x86/net/bpf_jit_comp.c

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