linux-kernel - Re: [PATCH] mm: relax ptrace mode in process_vm

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Message-ID: <20180306174427.GB2080@avx2>
Date:   Tue, 6 Mar 2018 20:44:27 +0300
From:   Alexey Dobriyan <adobriyan@...il.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     linux-kernel@...r.kernel.org, yeohc@....ibm.com, jann@...jh.net
Subject: Re: [PATCH] mm: relax ptrace mode in process_vm_readv(2)

On Mon, Mar 05, 2018 at 04:07:10PM -0800, Andrew Morton wrote:
> On Sat, 3 Mar 2018 23:11:16 +0300 Alexey Dobriyan <adobriyan@...il.com> wrote:
> 
> > It is more natural to check for read-from-memory permissions in case of
> > process_vm_readv() as PTRACE_MODE_ATTACH is equivalent to write
> > permissions.
> > 
> > ...
> >
> > --- a/mm/process_vm_access.c
> > +++ b/mm/process_vm_access.c
> > @@ -204,7 +204,7 @@ static ssize_t process_vm_rw_core(pid_t pid, struct iov_iter *iter,
> >  		goto free_proc_pages;
> >  	}
> >  
> > -	mm = mm_access(task, PTRACE_MODE_ATTACH_REALCREDS);
> > +	mm = mm_access(task, vm_write ? PTRACE_MODE_ATTACH_REALCREDS : PTRACE_MODE_READ_REALCREDS);
> >  	if (!mm || IS_ERR(mm)) {
> >  		rc = IS_ERR(mm) ? PTR_ERR(mm) : -ESRCH;
> >  		/*
> 
> But what is the risk of breaking existing userspace?

Permissions for write/ATTACH should be more strict than for read/READ,
so loosening them should be fine.

Unless LSM does silly things of course.