Commit bc81426f authored May 31, 2019 by Michal Koutný Committed by Linus Torvalds Jun 01, 2019

prctl_set_mm: downgrade mmap_sem to read lock

The commit a3b609ef ("proc read mm's {arg,env}_{start,end} with mmap
semaphore taken.") added synchronization of reading argument/environment
boundaries under mmap_sem.  Later commit 88aa7cc6 ("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: 88aa7cc6 ("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>

parent 11bbd8b4

kernel/sys.c

+9 −2

Original line number	Diff line number	Diff line
		@@ -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;
		}

mm/util.c

+2 −2

Original line number	Diff line number	Diff line
		@@ -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;

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