Re: [PATCH v2] kprobes/x86: Use copy_from_kernel_nofault() to read from unsafe address

[Jinghao Jia]

On 3/14/24 10:17, Masami Hiramatsu (Google) wrote:
> From: Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>
> 
> Read from an unsafe address with copy_from_kernel_nofault() in
> arch_adjust_kprobe_addr() because this function is used before checking
> the address is in text or not. Syzcaller bot found a bug and reported
> the case if user specifies inaccessible data area,
> arch_adjust_kprobe_addr() will cause a kernel panic.

IMHO there is a check on the address in kallsyms_lookup_size_offset to see
if it is a kernel text address before arch_adjust_kprobe_addr is invoked.

The call chain is:

register_kprobe()
  _kprobe_addr()
    kallsyms_lookup_size_offset() <- check on addr is here
    arch_adjust_kprobe_addr()

I wonder why this check was not able to capture the problem in this bug
report (I cannot reproduce it locally).

Thanks,
--Jinghao

> 
> Reported-by: Qiang Zhang <zzqq0103.hey@xxxxxxxxx>
> Closes: https://urldefense.com/v3/__https://lore.kernel.org/all/CAKHoSAs2rof6vQVBw_Lg_j3QNku0CANZR2qmy4eT7R5Lo8MFbg@xxxxxxxxxxxxxx/__;!!DZ3fjg!_C9Dn6-GBlkyS2z34bDUBsEXkTkgWp45MNrd4Rl5I5slz2A3SrurXOxKupsxLMqxC2BMiySCTfB2-5fPhkLP1g$ 
> Fixes: cc66bb914578 ("x86/ibt,kprobes: Cure sym+0 equals fentry woes")
> Signed-off-by: Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>
> ---
>  Changes in v2:
>   - Fix to return NULL if failed to access the address.
> ---
>  arch/x86/kernel/kprobes/core.c |   11 ++++++++++-
>  1 file changed, 10 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kernel/kprobes/core.c b/arch/x86/kernel/kprobes/core.c
> index a0ce46c0a2d8..95e4ebe5d514 100644
> --- a/arch/x86/kernel/kprobes/core.c
> +++ b/arch/x86/kernel/kprobes/core.c
> @@ -335,7 +335,16 @@ static int can_probe(unsigned long paddr)
>  kprobe_opcode_t *arch_adjust_kprobe_addr(unsigned long addr, unsigned long offset,
>  					 bool *on_func_entry)
>  {
> -	if (is_endbr(*(u32 *)addr)) {
> +	u32 insn;
> +
> +	/*
> +	 * Since addr is not guaranteed to be safely accessed yet, use
> +	 * copy_from_kernel_nofault() to get the instruction.
> +	 */
> +	if (copy_from_kernel_nofault(&insn, (void *)addr, sizeof(u32)))
> +		return NULL;
> +
> +	if (is_endbr(insn)) {
>  		*on_func_entry = !offset || offset == 4;
>  		if (*on_func_entry)
>  			offset = 4;
>

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