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Subject[PATCH 3/3] prctl_set_mm: downgrade mmap_sem to read lock
Date
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).

v2: call find_vma without arg_lock held

CC: Cyrill Gorcunov <gorcunov@gmail.com>
CC: Laurent Dufour <ldufour@linux.ibm.com>
Signed-off-by: Michal Koutný <mkoutny@suse.com>
---
kernel/sys.c | 10 ++++++++--
1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/kernel/sys.c b/kernel/sys.c
index e1acb444d7b0..641fda756575 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -2123,9 +2123,14 @@ static int prctl_set_mm(int opt, unsigned long addr,

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;
@@ -2213,7 +2218,8 @@ 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
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