Re: [PATCH v4 1/2] procfs: Allow reading fdinfo with PTRACE_MODE_READ

[Randy Dunlap <rdunlap@infradead.org>]

On 2/5/21 1:33 PM, 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

Tangential:
Please just use ASCII "'" -- it's good enough.

> 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
> 
>  fs/proc/base.c |  4 ++--
>  fs/proc/fd.c   | 15 ++++++++++++++-
>  2 files changed, 16 insertions(+), 3 deletions(-)


-- 
~Randy