+ prctl_set_mm-downgrade-mmap_sem-to-read-lock.patch added to -mm tree

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The patch titled
     Subject: prctl_set_mm: downgrade mmap_sem to read lock
has been added to the -mm tree.  Its filename is
     prctl_set_mm-downgrade-mmap_sem-to-read-lock.patch

This patch should soon appear at
    http://ozlabs.org/~akpm/mmots/broken-out/prctl_set_mm-downgrade-mmap_sem-to-read-lock.patch
and later at
    http://ozlabs.org/~akpm/mmotm/broken-out/prctl_set_mm-downgrade-mmap_sem-to-read-lock.patch

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------------------------------------------------------
From: Michal Koutný <mkoutny@xxxxxxxx>
Subject: prctl_set_mm: downgrade mmap_sem to read lock

The commit a3b609ef9f8b ("proc read mm's {arg,env}_{start,end} with mmap
semaphore taken.") added synchronization of reading argument/environment
boundaries under mmap_sem.  Later commit 88aa7cc688d4 ("mm: introduce
arg_lock to protect arg_start|end and env_start|end in mm_struct") avoided
the coarse use of mmap_sem in similar situations.  But there still
remained two places that (mis)use mmap_sem.

get_cmdline should also use arg_lock instead of mmap_sem when it reads the
boundaries.

The second place that should use arg_lock is in prctl_set_mm.  By
protecting the boundaries fields with the arg_lock, we can downgrade
mmap_sem to reader lock (analogous to what we already do in
prctl_set_mm_map).

Link: http://lkml.kernel.org/r/20190502125203.24014-3-mkoutny@xxxxxxxx
Fixes: 88aa7cc688d4 ("mm: introduce arg_lock to protect arg_start|end and env_start|end in mm_struct")
Signed-off-by: Michal Koutný <mkoutny@xxxxxxxx>
Signed-off-by: Laurent Dufour <ldufour@xxxxxxxxxxxxx>
Co-developed-by: Laurent Dufour <ldufour@xxxxxxxxxxxxx>
Reviewed-by: Cyrill Gorcunov <gorcunov@xxxxxxxxx>
Acked-by: Michal Hocko <mhocko@xxxxxxxx>
Cc: Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx>
Cc: Mateusz Guzik <mguzik@xxxxxxxxxx>
Cc: Kirill Tkhai <ktkhai@xxxxxxxxxxxxx>
Cc: Konstantin Khlebnikov <khlebnikov@xxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---

 kernel/sys.c |   10 ++++++++--
 mm/util.c    |    4 ++--
 2 files changed, 10 insertions(+), 4 deletions(-)

--- a/kernel/sys.c~prctl_set_mm-downgrade-mmap_sem-to-read-lock
+++ a/kernel/sys.c
@@ -2127,9 +2127,14 @@ static int prctl_set_mm(int opt, unsigne
 
 	error = -EINVAL;
 
-	down_write(&mm->mmap_sem);
+	/*
+	 * arg_lock protects concurent updates of arg boundaries, we need mmap_sem for
+	 * a) concurrent sys_brk, b) finding VMA for addr validation.
+	 */
+	down_read(&mm->mmap_sem);
 	vma = find_vma(mm, addr);
 
+	spin_lock(&mm->arg_lock);
 	prctl_map.start_code	= mm->start_code;
 	prctl_map.end_code	= mm->end_code;
 	prctl_map.start_data	= mm->start_data;
@@ -2217,7 +2222,8 @@ static int prctl_set_mm(int opt, unsigne
 
 	error = 0;
 out:
-	up_write(&mm->mmap_sem);
+	spin_unlock(&mm->arg_lock);
+	up_read(&mm->mmap_sem);
 	return error;
 }
 
--- a/mm/util.c~prctl_set_mm-downgrade-mmap_sem-to-read-lock
+++ a/mm/util.c
@@ -717,12 +717,12 @@ int get_cmdline(struct task_struct *task
 	if (!mm->arg_end)
 		goto out_mm;	/* Shh! No looking before we're done */
 
-	down_read(&mm->mmap_sem);
+	spin_lock(&mm->arg_lock);
 	arg_start = mm->arg_start;
 	arg_end = mm->arg_end;
 	env_start = mm->env_start;
 	env_end = mm->env_end;
-	up_read(&mm->mmap_sem);
+	spin_unlock(&mm->arg_lock);
 
 	len = arg_end - arg_start;
 
_

Patches currently in -mm which might be from mkoutny@xxxxxxxx are

prctl_set_mm-refactor-checks-from-validate_prctl_map.patch
prctl_set_mm-downgrade-mmap_sem-to-read-lock.patch





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