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

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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@xxxxxxxxx> wrote:

Hi Suren,

On 10/22/22 00:32, Zach OKeefe wrote:
From: Zach O'Keefe <zokeefe@xxxxxxxxxx>

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@xxxxxxxxxx>
Cc: Minchan Kim <minchan@xxxxxxxxxx>
Signed-off-by: Zach O'Keefe <zokeefe@xxxxxxxxxx>

Reviewed-by: Suren Baghdasaryan <surenb@xxxxxxxxxx>

Thanks!  Patch applied.



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/>

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