Re: [PATCH v2] riscv: Only consider swbp/ss handlers for correct privileged mode

[Nam Cao]

On Sun, Aug 27, 2023 at 01:40:03PM +0200, Björn Töpel wrote:
> From: Björn Töpel <bjorn@xxxxxxxxxxxx>
> 
> RISC-V software breakpoint trap handlers are used for {k,u}probes.
> 
> When trapping from kernelmode, only the kernelmode handlers should be
> considered. Vice versa, only usermode handlers for usermode
> traps. This is not the case on RISC-V, which can trigger a bug if a
> userspace process uses uprobes, and a WARN() is triggered from
> kernelmode (which is implemented via {c.,}ebreak).
> 
> The kernel will trap on the kernelmode {c.,}ebreak, look for uprobes
> handlers, realize incorrectly that uprobes need to be handled, and
> exit the trap handler early. The trap returns to re-executing the
> {c.,}ebreak, and enter an infinite trap-loop.
> 
> The issue was found running the BPF selftest [1].
> 
> Fix this issue by only considering the swbp/ss handlers for
> kernel/usermode respectively. Also, move CONFIG ifdeffery from traps.c
> to the asm/{k,u}probes.h headers.
> 
> Note that linux/uprobes.h only include asm/uprobes.h if CONFIG_UPROBES
> is defined, which is why asm/uprobes.h needs to be unconditionally
> included in traps.c
> 
> Link: https://lore.kernel.org/linux-riscv/87v8d19aun.fsf@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/ # [1]
> Reported-by: kernel test robot <lkp@xxxxxxxxx>
> Closes: https://lore.kernel.org/oe-kbuild-all/202308271841.HlnnHFL7-lkp@xxxxxxxxx/
> Reviewed-by: Guo Ren <guoren@xxxxxxxxxx>
> Fixes: 74784081aac8 ("riscv: Add uprobes supported")
> Signed-off-by: Björn Töpel <bjorn@xxxxxxxxxxxx>

Reviewed-by: Nam Cao <namcaov@xxxxxxxxx>

Best regards,
Nam