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

[Liam R. Howlett]

..Add Shakeel's new email address

* Liam R. Howlett <Liam.Howlett@xxxxxxxxxx> [240913 10:31]:
> * Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx> [240913 10:06]:
> > 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@xxxxxxxxxx>
> 
> Probably needs a fixes 96cfe2c0fd23 tag?
> 
> Reviewed-by: Liam R. Howlett <Liam.Howlett@xxxxxxxxxx>
> 
> > ---
> >  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;
> >  	}
> > -- 
> > 2.46.0
> >