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@linux-foundation.org: coding style fixes]
Link: http://lkml.kernel.org/r/20190502125203.24014-3-mkoutny@suse.com
Fixes: 88aa7cc688d4 ("mm: introduce arg_lock to protect arg_start|end and env_start|end in mm_struct")
Signed-off-by: Michal Koutný <mkoutny@suse.com>
Signed-off-by: Laurent Dufour <ldufour@linux.ibm.com>
Co-developed-by: Laurent Dufour <ldufour@linux.ibm.com>
Reviewed-by: Cyrill Gorcunov <gorcunov@gmail.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Yang Shi <yang.shi@linux.alibaba.com>
Cc: Mateusz Guzik <mguzik@redhat.com>
Cc: Kirill Tkhai <ktkhai@virtuozzo.com>
Cc: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

2 files changed, 11 insertions, 4 deletions

diff --git a/kernel/sys.c b/kernel/sys.c
index 775bf8d18d03..2969304c29fe 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -2127,9 +2127,15 @@ 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;
@@ -2217,7 +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;
 }
 
diff --git a/mm/util.c b/mm/util.c
index 91682a2090ee..9834c4ab7d8e 100644
--- a/mm/util.c
+++ b/mm/util.c
@@ -718,12 +718,12 @@ int get_cmdline(struct task_struct *task, char *buffer, int buflen)
         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;