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

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Date:   Mon, 8 Mar 2021 13:06:46 -0500
From:   Kalesh Singh <kaleshsingh@...gle.com>
To:     Christian König <christian.koenig@....com>
Cc:     Jann Horn <jannh@...gle.com>,
        Jeffrey Vander Stoep <jeffv@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Minchan Kim <minchan@...nel.org>,
        Hridya Valsaraju <hridya@...gle.com>,
        Randy Dunlap <rdunlap@...radead.org>,
        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 Mon, Mar 8, 2021 at 12:54 PM Christian König
<christian.koenig@....com> wrote:
>
> Am 08.03.21 um 18:06 schrieb Kalesh Singh:
> > 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>
>
> Both patches are Acked-by: Christian König <christian.koenig@....com>

Thanks Christian.

>
> > ---
> > Hi everyone,
> >
> > The initial posting of this patch can be found at [1].
> > I didn't receive any feedback last time, so resending here.
> > Would really appreciate any constructive comments/suggestions.
> >
> > Thanks,
> > Kalesh
> >
> > [1] https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Flore.kernel.org%2Fr%2F20210208155315.1367371-1-kaleshsingh%40google.com%2F&amp;data=04%7C01%7Cchristian.koenig%40amd.com%7C38c98420f0564e15117f08d8e2549ff5%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637508200431130855%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&amp;sdata=deJBlAk6%2BEQkfAC8iRK95xhV1%2FiO9Si%2Bylc5Z0QzzrM%3D&amp;reserved=0
> >
> > Changes in v2:
> >    - Update patch description
> >   fs/proc/base.c |  4 ++--
> >   fs/proc/fd.c   | 15 ++++++++++++++-
> >   2 files changed, 16 insertions(+), 3 deletions(-)
> >
> > diff --git a/fs/proc/base.c b/fs/proc/base.c
> > index 3851bfcdba56..fd46d8dd0cf4 100644
> > --- a/fs/proc/base.c
> > +++ b/fs/proc/base.c
> > @@ -3159,7 +3159,7 @@ static const struct pid_entry tgid_base_stuff[] = {
> >       DIR("task",       S_IRUGO|S_IXUGO, proc_task_inode_operations, proc_task_operations),
> >       DIR("fd",         S_IRUSR|S_IXUSR, proc_fd_inode_operations, proc_fd_operations),
> >       DIR("map_files",  S_IRUSR|S_IXUSR, proc_map_files_inode_operations, proc_map_files_operations),
> > -     DIR("fdinfo",     S_IRUSR|S_IXUSR, proc_fdinfo_inode_operations, proc_fdinfo_operations),
> > +     DIR("fdinfo",     S_IRUGO|S_IXUGO, proc_fdinfo_inode_operations, proc_fdinfo_operations),
> >       DIR("ns",         S_IRUSR|S_IXUGO, proc_ns_dir_inode_operations, proc_ns_dir_operations),
> >   #ifdef CONFIG_NET
> >       DIR("net",        S_IRUGO|S_IXUGO, proc_net_inode_operations, proc_net_operations),
> > @@ -3504,7 +3504,7 @@ static const struct inode_operations proc_tid_comm_inode_operations = {
> >    */
> >   static const struct pid_entry tid_base_stuff[] = {
> >       DIR("fd",        S_IRUSR|S_IXUSR, proc_fd_inode_operations, proc_fd_operations),
> > -     DIR("fdinfo",    S_IRUSR|S_IXUSR, proc_fdinfo_inode_operations, proc_fdinfo_operations),
> > +     DIR("fdinfo",    S_IRUGO|S_IXUGO, proc_fdinfo_inode_operations, proc_fdinfo_operations),
> >       DIR("ns",        S_IRUSR|S_IXUGO, proc_ns_dir_inode_operations, proc_ns_dir_operations),
> >   #ifdef CONFIG_NET
> >       DIR("net",        S_IRUGO|S_IXUGO, proc_net_inode_operations, proc_net_operations),
> > diff --git a/fs/proc/fd.c b/fs/proc/fd.c
> > index 07fc4fad2602..6a80b40fd2fe 100644
> > --- a/fs/proc/fd.c
> > +++ b/fs/proc/fd.c
> > @@ -6,6 +6,7 @@
> >   #include <linux/fdtable.h>
> >   #include <linux/namei.h>
> >   #include <linux/pid.h>
> > +#include <linux/ptrace.h>
> >   #include <linux/security.h>
> >   #include <linux/file.h>
> >   #include <linux/seq_file.h>
> > @@ -72,6 +73,18 @@ static int seq_show(struct seq_file *m, void *v)
> >
> >   static int seq_fdinfo_open(struct inode *inode, struct file *file)
> >   {
> > +     bool allowed = false;
> > +     struct task_struct *task = get_proc_task(inode);
> > +
> > +     if (!task)
> > +             return -ESRCH;
> > +
> > +     allowed = ptrace_may_access(task, PTRACE_MODE_READ_FSCREDS);
> > +     put_task_struct(task);
> > +
> > +     if (!allowed)
> > +             return -EACCES;
> > +
> >       return single_open(file, seq_show, inode);
> >   }
> >
> > @@ -308,7 +321,7 @@ static struct dentry *proc_fdinfo_instantiate(struct dentry *dentry,
> >       struct proc_inode *ei;
> >       struct inode *inode;
> >
> > -     inode = proc_pid_make_inode(dentry->d_sb, task, S_IFREG | S_IRUSR);
> > +     inode = proc_pid_make_inode(dentry->d_sb, task, S_IFREG | S_IRUGO);
> >       if (!inode)
> >               return ERR_PTR(-ENOENT);
> >
>
> --
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