From bc81426f5beef7da863d3365bc9d45e820448745 Mon Sep 17 00:00:00 2001 From: =?utf8?q?Michal=20Koutn=C3=BD?= Date: Fri, 31 May 2019 22:30:19 -0700 Subject: [PATCH] prctl_set_mm: downgrade mmap_sem to read lock MIME-Version: 1.0 Content-Type: text/plain; charset=utf8 Content-Transfer-Encoding: 8bit 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 Signed-off-by: Laurent Dufour Co-developed-by: Laurent Dufour Reviewed-by: Cyrill Gorcunov Acked-by: Michal Hocko Cc: Yang Shi Cc: Mateusz Guzik Cc: Kirill Tkhai Cc: Konstantin Khlebnikov Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- kernel/sys.c | 11 +++++++++-- mm/util.c | 4 ++-- 2 files changed, 11 insertions(+), 4 deletions(-) diff --git a/kernel/sys.c b/kernel/sys.c index 775bf8d..2969304 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 91682a2..9834c4a 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; -- 2.7.4