[PATCH] prctl_set_mm: downgrade mmap_sem to read lock

["Michal Koutný" <mkoutny@suse.com>]

I learnt, it's, alas, too late to drop the non PRCTL_SET_MM_MAP calls
[1], so at least downgrade the write acquisition of mmap_sem as in the
patch below (that should be stacked on the previous one or squashed).

Cyrill, you mentioned lock changes in [1] but the link seems empty. Is
it supposed to be [2]? That could be an alternative to this patch after
some refreshments and clarifications.


[1] https://lore.kernel.org/lkml/20190417165632.GC3040@uranus.lan/
[2] https://lore.kernel.org/lkml/20180507075606.870903028@gmail.com/

========

Since commit 88aa7cc688d4 ("mm: introduce arg_lock to protect
arg_start|end and env_start|end in mm_struct") we use arg_lock for
boundaries modifications. Synchronize prctl_set_mm with this lock and
keep mmap_sem for reading only (analogous to what we already do in
prctl_set_mm_map).

Also, save few cycles by looking up VMA only after performing basic
arguments validation.

Signed-off-by: Michal Koutný <mkoutny@suse.com>
---
 kernel/sys.c | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/kernel/sys.c b/kernel/sys.c
index 12df0e5434b8..bbce0f26d707 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -2125,8 +2125,12 @@ static int prctl_set_mm(int opt, unsigned long addr,
 
 	error = -EINVAL;
 
-	down_write(&mm->mmap_sem);
-	vma = find_vma(mm, addr);
+	/*
+	 * 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);
+	spin_lock(&mm->arg_lock);
 
 	prctl_map.start_code	= mm->start_code;
 	prctl_map.end_code	= mm->end_code;
@@ -2185,6 +2189,7 @@ static int prctl_set_mm(int opt, unsigned long addr,
 	if (error)
 		goto out;
 
+	vma = find_vma(mm, addr);
 	switch (opt) {
 	/*
 	 * If command line arguments and environment
@@ -2218,7 +2223,8 @@ static int prctl_set_mm(int opt, unsigned long addr,
 
 	error = 0;
 out:
-	up_write(&mm->mmap_sem);
+	spin_unlock(&mm->arg_lock);
+	up_read(&mm->mmap_sem);
 	return error;
 }
 
-- 
2.16.4