linux-kernel - Re: [RESEND PATCH v6 1/2] procfs: Allow reading fdinfo with PTRACE_MODE

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Date:   Wed, 17 Mar 2021 23:14:08 -0700
From:   Kalesh Singh <kaleshsingh@...gle.com>
To:     Kees Cook <keescook@...omium.org>
Cc:     Jann Horn <jannh@...gle.com>,
        Jeffrey Vander Stoep <jeffv@...gle.com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Minchan Kim <minchan@...nel.org>,
        Hridya Valsaraju <hridya@...gle.com>,
        Randy Dunlap <rdunlap@...radead.org>,
        Christian König <christian.koenig@....com>,
        Matthew Wilcox <willy@...radead.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        "Cc: Android Kernel" <kernel-team@...roid.com>,
        Alexey Dobriyan <adobriyan@...il.com>,
        Jonathan Corbet <corbet@....net>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mauro Carvalho Chehab <mchehab+huawei@...nel.org>,
        Michal Hocko <mhocko@...e.com>,
        Alexey Gladkov <gladkov.alexey@...il.com>,
        Szabolcs Nagy <szabolcs.nagy@....com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Michel Lespinasse <walken@...gle.com>,
        Bernd Edlinger <bernd.edlinger@...mail.de>,
        Andrei Vagin <avagin@...il.com>, Helge Deller <deller@....de>,
        James Morris <jamorris@...ux.microsoft.com>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>
Subject: Re: [RESEND PATCH v6 1/2] procfs: Allow reading fdinfo with PTRACE_MODE_READ

On Wed, Mar 17, 2021 at 10:55 PM Kees Cook <keescook@...omium.org> wrote:
>
> On Mon, Mar 08, 2021 at 05:06:40PM +0000, Kalesh Singh wrote:
> > Android captures per-process system memory state when certain low memory
> > events (e.g a foreground app kill) occur, to identify potential memory
> > hoggers. In order to measure how much memory a process actually consumes,
> > it is necessary to include the DMA buffer sizes for that process in the
> > memory accounting. Since the handle to DMA buffers are raw FDs, it is
> > important to be able to identify which processes have FD references to
> > a DMA buffer.
> >
> > Currently, DMA buffer FDs can be accounted using /proc/<pid>/fd/* and
> > /proc/<pid>/fdinfo -- both are only readable by the process owner,
> > as follows:
> >   1. Do a readlink on each FD.
> >   2. If the target path begins with "/dmabuf", then the FD is a dmabuf FD.
> >   3. stat the file to get the dmabuf inode number.
> >   4. Read/ proc/<pid>/fdinfo/<fd>, to get the DMA buffer size.
> >
> > Accessing other processes' fdinfo requires root privileges. This limits
> > the use of the interface to debugging environments and is not suitable
> > for production builds.  Granting root privileges even to a system process
> > increases the attack surface and is highly undesirable.
> >
> > Since fdinfo doesn't permit reading process memory and manipulating
> > process state, allow accessing fdinfo under PTRACE_MODE_READ_FSCRED.
> >
> > Suggested-by: Jann Horn <jannh@...gle.com>
> > Signed-off-by: Kalesh Singh <kaleshsingh@...gle.com>
>
> Reviewed-by: Kees Cook <keescook@...omium.org>
>
> Who would be best to pick this up? Maybe akpm?

Thanks Kees. Andrew has queued the patchset for the mm tree.

>
> --
> Kees Cook