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).
[akpm@xxxxxxxxxxxxxxxxxxxx: coding style fixes]
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 | 11 +++++++++--
mm/util.c | 4 ++--
2 files changed, 11 insertions(+), 4 deletions(-)
--- a/kernel/sys.c~prctl_set_mm-downgrade-mmap_sem-to-read-lock
+++ a/kernel/sys.c
@@ -2127,9 +2127,15 @@ 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 +2223,8 @@ 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
@@ -718,12 +718,12 @@ 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;
_
