From: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
[ Upstream commit 90fb702791bf99b959006972e8ee7bb4609f441b ]
autofs got broken in some configurations by commit 13c164b1a186
("autofs: switch to kernel_write") because there is now an extra LSM
permission check done by security_file_permission() in rw_verify_area().
autofs is one if the few places that really does want the much more
limited __kernel_write(), because the write is an internal kernel one
that shouldn't do any user permission checks (it also doesn't need the
file_start_write/file_end_write logic, since it's just a pipe).
There are a couple of other cases like that - accounting, core dumping,
and splice - but autofs stands out because it can be built as a module.
As a result, we need to export this internal __kernel_write() function
again.
We really don't want any other module to use this, but we don't have a
"EXPORT_SYMBOL_FOR_AUTOFS_ONLY()". But we can mark it GPL-only to at
least approximate that "internal use only" for licensing.
While in this area, make autofs pass in NULL for the file position
pointer, since it's always a pipe, and we now use a NULL file pointer
for streaming file descriptors (see file_ppos() and commit 438ab720c675:
"vfs: pass ppos=NULL to .read()/.write() of FMODE_STREAM files")
This effectively reverts commits 9db977522449 ("fs: unexport
__kernel_write") and 13c164b1a186 ("autofs: switch to kernel_write").
Fixes: 13c164b1a186 ("autofs: switch to kernel_write")
Reported-by: Ondrej Mosnacek <omosnace@xxxxxxxxxx>
Acked-by: Christoph Hellwig <hch@xxxxxx>
Acked-by: Acked-by: Ian Kent <raven@xxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---
fs/autofs/waitq.c | 2 +-
fs/read_write.c | 8 ++++++++
2 files changed, 9 insertions(+), 1 deletion(-)
diff --git a/fs/autofs/waitq.c b/fs/autofs/waitq.c
index 74c886f7c51cb..5ced859dac539 100644
--- a/fs/autofs/waitq.c
+++ b/fs/autofs/waitq.c
@@ -53,7 +53,7 @@ static int autofs_write(struct autofs_sb_info *sbi,
mutex_lock(&sbi->pipe_mutex);
while (bytes) {
- wr = kernel_write(file, data, bytes, &file->f_pos);
+ wr = __kernel_write(file, data, bytes, NULL);
if (wr <= 0)
break;
data += wr;
diff --git a/fs/read_write.c b/fs/read_write.c
index 4fb797822567a..9a5cb9c2f0d46 100644
--- a/fs/read_write.c
+++ b/fs/read_write.c
@@ -538,6 +538,14 @@ ssize_t __kernel_write(struct file *file, const void *buf, size_t count, loff_t
inc_syscw(current);
return ret;
}
+/*
+ * This "EXPORT_SYMBOL_GPL()" is more of a "EXPORT_SYMBOL_DONTUSE()",
+ * but autofs is one of the few internal kernel users that actually
+ * wants this _and_ can be built as a module. So we need to export
+ * this symbol for autofs, even though it really isn't appropriate
+ * for any other kernel modules.
+ */
+EXPORT_SYMBOL_GPL(__kernel_write);
ssize_t kernel_write(struct file *file, const void *buf, size_t count,
loff_t *pos)
--
2.25.1
