Re: [PATCH] mm/madvise: process_madvise() drop capability check if same mm

[Vlastimil Babka <vbabka@suse.cz>]

On 9/13/24 16:06, Lorenzo Stoakes wrote:
> In commit 96cfe2c0fd23 ("mm/madvise: replace ptrace attach requirement for
> process_madvise") process_madvise() was updated to require the caller to
> possess the CAP_SYS_NICE capability to perform the operation, in addition
> to a check against PTRACE_MODE_READ performed by mm_access().
> 
> The mm_access() function explicitly checks to see if the address space of
> the process being referenced is the current one, in which case no check is
> performed.
> 
> We, however, do not do this when checking the CAP_SYS_NICE capability. This
> means that we insist on the caller possessing this capability in order to
> perform madvise() operations on its own address space, which seems
> nonsensical.
> 
> Simply add a check to allow for an invocation of this function with pidfd
> set to the current process without elevation.
> 
> Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>

Acked-by: Vlastimil Babka <vbabka@suse.cz>

> ---
>  mm/madvise.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/madvise.c b/mm/madvise.c
> index 4e64770be16c..ff139e57cca2 100644
> --- a/mm/madvise.c
> +++ b/mm/madvise.c
> @@ -1520,7 +1520,7 @@ SYSCALL_DEFINE5(process_madvise, int, pidfd, const struct iovec __user *, vec,
>  	 * Require CAP_SYS_NICE for influencing process performance. Note that
>  	 * only non-destructive hints are currently supported.
>  	 */
> -	if (!capable(CAP_SYS_NICE)) {
> +	if (mm != current->mm && !capable(CAP_SYS_NICE)) {
>  		ret = -EPERM;
>  		goto release_mm;
>  	}