Re: [PATCHv2] src/listxattr: Fix reading past the end of the user buffer

[Nikolay Borisov <nborisov@suse.com>]

On 30.05.2017 17:10, Nikolay Borisov wrote:
> listxattr reaturns a null-terminated list of entries that represent
> the xattr names. However, if it is passed larger buffer than it
> requires it won't zero-out the rest of the memory. The way the
> loop iterator in listxattr.c is written makes it go print every
> null-terminated entry up to bufsize (which is user passed parameter).
> This can lead to a situation where listxattr users N bytes out of
> M bytes big buffer ( M > N). This will leave the rest (M-N)
> as garbage, which in turn will be printed by listxattr. Fix this
> by converting the 'for' loop to 'while' and properly ensuring
> we are reading at most howevermany elements the syscall reported
> it returned
> 
> Signed-off-by: Nikolay Borisov <nborisov@suse.com>
> ---
> 
> v2: 
>  Rewrite the loop, hopefully making the code a bit more legible.
>  Functionally it's the same as my previous fix
> 
>  src/listxattr.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/src/listxattr.c b/src/listxattr.c
> index cd46637a..584ebb2d 100644
> --- a/src/listxattr.c
> +++ b/src/listxattr.c
> @@ -60,10 +60,10 @@ int main(int argc, char **argv)
>          if (ret < 0) {
>                  perror("listxattr");
>          } else {
> -                char *l;
> -                for (l = buf; l != (buf + bufsize) && *l != '\0';
> -                                l = strchr(l, '\0') + 1) {
> +                char *l = buf;
> +                while (l != (buf + ret)) {

Actually I believe while (l < (buf + ret)) is the correct form.

>                          printf("xattr: %s\n", l);
> +                        l = strchr(l, '\0') + 1;
>                  }
>          }
>  
>