In the Linux kernel, the following vulnerability has been resolved: bpf: Reject load-acquire from pointers requiring fault protection A BPF_LOAD_ACQ is not rewritten to a BPF_PROBE_MEM load by the verifier, unlike a regular BPF_LDX, so…

Overview

In the Linux kernel, the following vulnerability has been resolved:

bpf: Reject load-acquire from pointers requiring fault protection

A BPF_LOAD_ACQ is not rewritten to a BPF_PROBE_MEM load by the verifier, unlike a regular BPF_LDX, so the JIT emits a plain load with no exception table entry and a fault panics the kernel instead of being handled.

Reject the source pointer types that a BPF_LDX would have had that fault protection applied to, i.e. the ones bpf_convert_ctx_accesses() turns into BPF_PROBE_MEM: a bare PTR_TO_BTF_ID, PTR_TO_BTF_ID | PTR_UNTRUSTED, PTR_TO_BTF_ID | MEM_ALLOC | PTR_UNTRUSTED and PTR_TO_MEM | MEM_RDONLY | PTR_UNTRUSTED.

This is reachable e.g. by loading ->mm out of a trusted task_struct yields an untrusted pointer to mm_struct, and it is NULL for a kernel thread:

[...] SEC("tp_btf/sched_switch") int BPF_PROG(demo, bool preempt, struct task_struct *prev, struct task_struct *next) { struct mm_struct mm = next->mm; / untrusted */

  out_ldx = (__u64)mm->pgd;         /* BPF_LDX      */
  out_acq = load_acquire(&mm->pgd); /* BPF_LOAD_ACQ */
  return 0;

} [...]

Both dereference the same pointer, but only the BPF_LDX is protected (x86-64 JIT, jump targets shown prog-relative):

[...] ; out_ldx = (__u64)mm->pgd; 17: movq $-10485760, %r10 1e: movq %rsi, %r11 21: addq $184, %r11 28: subq %r10, %r11 2b: movabsq $140737498841088, %r10 35: cmpq %r10, %r11 38: ja 0x3e <-- kernel addr? 3a: xorl %edi, %edi <-- no: dst = 0, skip the load 3c: jmp 0x45 3e: movq 184(%rsi), %rdi <-- yes: load + extable entry [...] ; load_acquire(&mm->pgd) 53: movq %rsi, %rdi 56: movq 184(%rdi), %rax <-- no check, no extable entry [...]

Note that BPF_PROBE_MEM is not visible in a bpftool xlated dump, as bpf_insn_prepare_dump() rewrites it back to BPF_MEM.

A PTR_TRUSTED pointer is deliberately not on the list. Such a load is not converted either, but it does not need to be, since the pointer is guaranteed live, so load-acquire from it stays allowed.

The check is gated on BPF_LOAD_ACQ so that atomic RMW and store-release error messages are unchanged; writes (RMW / store-release) to such pointers are already rejected elsewhere, so only load-acquire needs this.

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.