Re: [PATCH] uprobes/x86: fix detection of 32-bit user mode

[Sebastian Mayr]

On Sun, 2019-07-28 at 17:26 +0200, Sebastian Mayr wrote:
> 32-bit processes running on a 64-bit kernel are not always detected
> correctly, causing the process to crash when uretprobes are
> installed.
> The reason for the crash is that in_ia32_syscall() is used to
> determine
> the process's mode, which only works correctly when called from a
> syscall. In the case of uretprobes, however, the function is called
> from
> a software interrupt and always returns 'false' (on a 64-bit kernel).
> In
> consequence this leads to corruption of the process's return address.
> 
> This can be fixed by using user_64bit_mode(), which should always be
> correct.
> 
> Signed-off-by: Sebastian Mayr <me@xxxxxx>
> ---
> 
> Please note that I just stumbled over this bug and am not really
> familiar with all the internals. So take the patch and, in
> particular,
> the commit message with a grain of salt. Thanks!
> 
>  arch/x86/kernel/uprobes.c | 14 +++++++-------
>  1 file changed, 7 insertions(+), 7 deletions(-)
> 
> diff --git a/arch/x86/kernel/uprobes.c b/arch/x86/kernel/uprobes.c
> index 918b5092a85f..d6e261202c6b 100644
> --- a/arch/x86/kernel/uprobes.c
> +++ b/arch/x86/kernel/uprobes.c
> @@ -508,9 +508,9 @@ struct uprobe_xol_ops {
>  	void	(*abort)(struct arch_uprobe *, struct pt_regs *);
>  };
>  
> -static inline int sizeof_long(void)
> +static inline int sizeof_long(struct pt_regs *regs)
>  {
> -	return in_ia32_syscall() ? 4 : 8;
> +	return user_64bit_mode(regs) ? 8 : 4;
>  }
>  
>  static int default_pre_xol_op(struct arch_uprobe *auprobe, struct
> pt_regs *regs)
> @@ -521,9 +521,9 @@ static int default_pre_xol_op(struct arch_uprobe
> *auprobe, struct pt_regs *regs)
>  
>  static int emulate_push_stack(struct pt_regs *regs, unsigned long
> val)
>  {
> -	unsigned long new_sp = regs->sp - sizeof_long();
> +	unsigned long new_sp = regs->sp - sizeof_long(regs);
>  
> -	if (copy_to_user((void __user *)new_sp, &val, sizeof_long()))
> +	if (copy_to_user((void __user *)new_sp, &val,
> sizeof_long(regs)))
>  		return -EFAULT;
>  
>  	regs->sp = new_sp;
> @@ -556,7 +556,7 @@ static int default_post_xol_op(struct arch_uprobe
> *auprobe, struct pt_regs *regs
>  		long correction = utask->vaddr - utask->xol_vaddr;
>  		regs->ip += correction;
>  	} else if (auprobe->defparam.fixups & UPROBE_FIX_CALL) {
> -		regs->sp += sizeof_long(); /* Pop incorrect return
> address */
> +		regs->sp += sizeof_long(regs); /* Pop incorrect return
> address */
>  		if (emulate_push_stack(regs, utask->vaddr + auprobe-
> >defparam.ilen))
>  			return -ERESTART;
>  	}
> @@ -675,7 +675,7 @@ static int branch_post_xol_op(struct arch_uprobe
> *auprobe, struct pt_regs *regs)
>  	 * "call" insn was executed out-of-line. Just restore ->sp and
> restart.
>  	 * We could also restore ->ip and try to call
> branch_emulate_op() again.
>  	 */
> -	regs->sp += sizeof_long();
> +	regs->sp += sizeof_long(regs);
>  	return -ERESTART;
>  }
>  
> @@ -1056,7 +1056,7 @@ bool arch_uprobe_skip_sstep(struct arch_uprobe
> *auprobe, struct pt_regs *regs)
>  unsigned long
>  arch_uretprobe_hijack_return_addr(unsigned long trampoline_vaddr,
> struct pt_regs *regs)
>  {
> -	int rasize = sizeof_long(), nleft;
> +	int rasize = sizeof_long(regs), nleft;
>  	unsigned long orig_ret_vaddr = 0; /* clear high bits for 32-bit 
> apps */
>  
>  	if (copy_from_user(&orig_ret_vaddr, (void __user *)regs->sp,
> rasize))

Any feedback on this patch would be greatly appreciated.

Thanks,
Sebastian