Re: [PATCH v6 1/2] procfs: Allow reading fdinfo with PTRACE_MODE_READ

[Kalesh Singh <kaleshsingh@google.com>]

On Mon, Feb 8, 2021 at 10:53 AM Kalesh Singh <kaleshsingh@google.com> 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@google.com>
> Signed-off-by: Kalesh Singh <kaleshsingh@google.com>
> ---
> Changes in v2:
>   - Update patch description

Hi all,

Kindly requesting maintainers to take a look at this patch set.

Thanks,
Kalesh
>
>  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 b3422cda2a91..a37f9de7103f 100644
> --- a/fs/proc/base.c
> +++ b/fs/proc/base.c
> @@ -3160,7 +3160,7 @@ 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 cb51763ed554..585e213301f9 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);
>  }
>
> @@ -307,7 +320,7 @@ 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);
>
> --
> 2.30.0.478.g8a0d178c01-goog
>