bpf, x86: emit KASAN checks in x86 JITed programs

Insert KASAN shadow memory checks before memory load and store
operations in JIT-compiled BPF programs. This helps detect memory safety
bugs such as use-after-free and out-of-bounds accesses at runtime.

The main instructions being targeted are BPF_ST, BPF_STX and BPF_LDX,
but not all of them are being instrumented:
- if the load/store instruction is in fact accessing the program stack,
  emit_kasan_check silently skips the instrumentation, as we can already
  benefit from guard pages to monitor stack accesses.
- if the load/store instruction is a BPF_PROBE_MEM or a BPF_PROBE_ATOMIC
  instruction, we do not instrument it, as the passed address can fault
  (hence the custom fault management with BPF_PROBE_XXX instructions),
  and so the corresponding kasan check could fault as well.

To support those new instructions insertion, create the
emit_kasan_check() helper that emits KASAN shadow memory checks before
memory accesses in JIT-compiled BPF programs. The implementation relies
on the existing __asan_{load,store}X functions from KASAN subsystem. The
helper:
- saves registers. This includes caller-saved registers, but also
  temporary registers, as those were possibly used by the
  affected program.
- computes the accessed address and stores it in %rdi
- calls the relevant function, depending on the instruction being a load
  or a store, and the size of the access.
- restores registers

The special care needed when inserting this instrumentation comes at the
cost of a non negligeable increase in JITed code size. For example, a
bare

  mov 	0x0(%si),rbx # Load in rbx content at address stored in rsi

becomes

  push    %rax
  push    %rcx
  push    %rdx
  push    %rsi
  push    %rdi
  push    %r8
  push    %r9
  push    %r10
  push    %r11
  mov     %rsi,%rdi
  call    0xffffffff81da0a60 <__asan_load8>
  pop     %r11
  pop     %r10
  pop     %r9
  pop     %r8
  pop     %rdi
  pop     %rsi
  pop     %rdx
  pop     %rcx
  pop     %rax
  mov     0x0(%rsi),rbx

Signed-off-by: Alexis Lothoré (eBPF Foundation) <alexis.lothore@bootlin.com>
Link: https://lore.kernel.org/r/20260903-kasan-v9-4-2407fe99255a@bootlin.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

0 files changed, 0 insertions, 0 deletions