fs: use acquire ordering in __fget_light()
[ Upstream commit 7ee47dcfff
]
We must prevent the CPU from reordering the files->count read with the
FD table access like this, on architectures where read-read reordering is
possible:
files_lookup_fd_raw()
close_fd()
put_files_struct()
atomic_read(&files->count)
I would like to mark this for stable, but the stable rules explicitly say
"no theoretical races", and given that the FD table pointer and
files->count are explicitly stored in the same cacheline, this sort of
reordering seems quite unlikely in practice...
Signed-off-by: Jann Horn <jannh@google.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Sasha Levin <sashal@kernel.org>
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parent
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commit
66717ad03b
11
fs/file.c
11
fs/file.c
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@ -1029,7 +1029,16 @@ static unsigned long __fget_light(unsigned int fd, fmode_t mask)
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struct files_struct *files = current->files;
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struct file *file;
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if (atomic_read(&files->count) == 1) {
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/*
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* If another thread is concurrently calling close_fd() followed
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* by put_files_struct(), we must not observe the old table
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* entry combined with the new refcount - otherwise we could
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* return a file that is concurrently being freed.
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*
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* atomic_read_acquire() pairs with atomic_dec_and_test() in
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* put_files_struct().
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*/
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if (atomic_read_acquire(&files->count) == 1) {
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file = files_lookup_fd_raw(files, fd);
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if (!file || unlikely(file->f_mode & mask))
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return 0;
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