Re: proc: fix /proc/*/map_files lookup

[Andrei Vagin <avagin@virtuozzo.com>]

On Tue, Nov 21, 2017 at 12:27:06AM +0300, Alexey Dobriyan wrote:
> Current code does:
> 
> 	if (sscanf(dentry->d_name.name, "%lx-%lx", start, end) != 2)
> 
> However sscanf() is broken garbage.
> 
> It silently accepts whitespace between format specifiers
> (did you know that?).
> 
> It silently accepts valid strings which result in integer overflow.
> 
> Do not use sscanf() for any even remotely reliable parsing code.

This patch breaks criu, criu has one places where a file name is generated
as map_files/%p-%p

openat(1048572, "map_files/0x7f9912dd5000-0x7f9912de4000", O_RDWR) = -1 ENOENT (No such file or directory) <0.000015>

And this code worked before this patch and it doesn't work with this
patch. And you have to know that we never break user-space programs ;)

But seriously, the patch looks good to me, but I would prefer to not queue
it into stable kernels.

Thanks,
Andrei


> 
> 	OK
> 	# readlink '/proc/1/map_files/55a23af39000-55a23b05b000'
> 	/lib/systemd/systemd
> 
> 	broken
> 	# readlink '/proc/1/map_files/               55a23af39000-55a23b05b000'
> 	/lib/systemd/systemd
> 
> 	broken
> 	# readlink '/proc/1/map_files/55a23af39000-55a23b05b000    '
> 	/lib/systemd/systemd
> 
> 	very broken
> 	# readlink '/proc/1/map_files/1000000000000000055a23af39000-55a23b05b000'
> 	/lib/systemd/systemd
> 
> Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
> Cc: stable@kernel.org
> ---
> 
>  fs/proc/base.c |   29 ++++++++++++++++++++++++++++-
>  1 file changed, 28 insertions(+), 1 deletion(-)
> 
> --- a/fs/proc/base.c
> +++ b/fs/proc/base.c
> @@ -100,6 +100,8 @@
>  #include "internal.h"
>  #include "fd.h"
>  
> +#include "../../lib/kstrtox.h"
> +
>  /* NOTE:
>   *	Implementing inode permission operations in /proc is almost
>   *	certainly an error.  Permission checks need to happen during
> @@ -1907,8 +1909,33 @@ bool proc_fill_cache(struct file *file, struct dir_context *ctx,
>  static int dname_to_vma_addr(struct dentry *dentry,
>  			     unsigned long *start, unsigned long *end)
>  {
> -	if (sscanf(dentry->d_name.name, "%lx-%lx", start, end) != 2)
> +	const char *str = dentry->d_name.name;
> +	unsigned long long sval, eval;
> +	unsigned int len;
> +
> +	len = _parse_integer(str, 16, &sval);
> +	if (len & KSTRTOX_OVERFLOW)
> +		return -EINVAL;
> +	if (sval != (unsigned long)sval)
> +		return -EINVAL;
> +	str += len;
> +
> +	if (*str != '-')
>  		return -EINVAL;
> +	str++;
> +
> +	len = _parse_integer(str, 16, &eval);
> +	if (len & KSTRTOX_OVERFLOW)
> +		return -EINVAL;
> +	if (eval != (unsigned long)eval)
> +		return -EINVAL;
> +	str += len;
> +
> +	if (*str != '\0')
> +		return -EINVAL;
> +
> +	*start = sval;
> +	*end = eval;
>  
>  	return 0;
>  }