Re: [PATCH man-pages v3 3/4] process_madvise.2: fix capability and ptrace requirements

["Zach O'Keefe" <zokeefe@google.com>]

On Mon, Oct 31, 2022 at 1:24 PM Alejandro Colomar
<alx.manpages@gmail.com> wrote:
>
> Hi Suren, Zach,
>
> On 10/31/22 20:13, Suren Baghdasaryan wrote:
> > Hi Alex,
> >
> > On Sun, Oct 30, 2022 at 4:50 AM Alejandro Colomar
> > <alx.manpages@gmail.com> wrote:
> >>
> >> Hi Suren,
> >>
> >> On 10/22/22 00:32, Zach OKeefe wrote:
> >>> From: Zach O'Keefe <zokeefe@google.com>
> >>>
> >>> The initial commit of process_madvise(2) to Linux, commit ecb8ac8b1f14
> >>> ("mm/madvise: introduce process_madvise() syscall: an external memory
> >>> hinting API"), relied on PTRACE_MODE_ATTACH_FSCREDS (see ptrace(2)),
> >>> but was amended by commit 96cfe2c0fd23 ("mm/madvise: replace ptrace
> >>> attach requirement for process_madvise") which replaced this with a
> >>> combination of PTRACE_MODE_READ and CAP_SYS_NICE (PTRACE_MODE_READ to
> >>> prevent leaking ASLR metadata and CAP_SYS_NICE for influencing process
> >>> performance).
> >>>
> >>> The initial commit of process_madvise(2) to man-pages project, made
> >>> after the second patch, included two errors:
> >>>
> >>> 1) CAP_SYS_ADMIN instead of CAP_SYS_NICE
> >>> 2) PTRACE_MODE_READ_REALCREDS instead of PTRACE_MODE_READ_FSCREDS
> >>>
> >>> Correct this in the man-page for process_madvise(2).
> >>>
> >>> Fixes: a144f458b ("process_madvise.2: Document process_madvise(2)")
> >>> Cc: Suren Baghdasaryan <surenb@google.com>
> >>> Cc: Minchan Kim <minchan@kernel.org>
> >>> Signed-off-by: Zach O'Keefe <zokeefe@google.com>
> >
> > Reviewed-by: Suren Baghdasaryan <surenb@google.com>
>
> Thanks!  Patch applied.

Thanks Suren & Alex!

Best,
Zach

> >
> >>
> >> You added your Reviewed-by tag to v2 of this patch.  I guess you'd like to put
> >> it in this one too, but since it changed slightly, I'd like you to confirm.
> >
> > Thanks for the reminder!
>
> :)
>
> Cheers,
>
> Alex
>
> > Suren.
> >
> >>
> >> Thanks,
> >>
> >> Alex
> >>
> >>> ---
> >>>    man2/process_madvise.2 | 21 +++++++++++++++++----
> >>>    1 file changed, 17 insertions(+), 4 deletions(-)
> >>>
> >>> diff --git a/man2/process_madvise.2 b/man2/process_madvise.2
> >>> index 6208206e4..44d3b94e8 100644
> >>> --- a/man2/process_madvise.2
> >>> +++ b/man2/process_madvise.2
> >>> @@ -105,16 +105,20 @@ remote process.
> >>>    No further elements will be processed beyond that point.
> >>>    (See the discussion regarding partial advice in RETURN VALUE.)
> >>>    .PP
> >>> -Permission to apply advice to another process is governed by a
> >>> +.\" commit 96cfe2c0fd23ea7c2368d14f769d287e7ae1082e
> >>> +Starting in Linux 5.12,
> >>> +permission to apply advice to another process is governed by
> >>>    ptrace access mode
> >>> -.B PTRACE_MODE_READ_REALCREDS
> >>> +.B PTRACE_MODE_READ_FSCREDS
> >>>    check (see
> >>>    .BR ptrace (2));
> >>>    in addition,
> >>>    because of the performance implications of applying the advice,
> >>>    the caller must have the
> >>> -.B CAP_SYS_ADMIN
> >>> -capability.
> >>> +.B CAP_SYS_NICE
> >>> +capability
> >>> +(see
> >>> +.BR capabilities (7)).
> >>>    .SH RETURN VALUE
> >>>    On success,
> >>>    .BR process_madvise ()
> >>> @@ -180,6 +184,15 @@ configuration option.
> >>>    The
> >>>    .BR process_madvise ()
> >>>    system call is Linux-specific.
> >>> +.SH NOTES
> >>> +When this system call first appeared in Linux 5.10,
> >>> +permission to apply advice to another process was entirely governed by
> >>> +ptrace access mode
> >>> +.B PTRACE_MODE_ATTACH_FSCREDS
> >>> +check (see
> >>> +.BR ptrace (2)).
> >>> +This requirement was relaxed in Linux 5.12 so that the caller didn't require
> >>> +full control over the target process.
> >>>    .SH SEE ALSO
> >>>    .BR madvise (2),
> >>>    .BR pidfd_open (2),
> >>
> >> --
> >> <http://www.alejandro-colomar.es/>
>
> --
> <http://www.alejandro-colomar.es/>