[PATCH 6.4 706/800] afs: Fix accidental truncation when storing data

Greg Kroah-Hartman gregkh at linuxfoundation.org
Sun Jul 16 12:49:19 PDT 2023


From: David Howells <dhowells at redhat.com>

[ Upstream commit 03275585cabd0240944f19f33d7584a1b099a3a8 ]

When an AFS FS.StoreData RPC call is made, amongst other things it is
given the resultant file size to be.  On the server, this is processed
by truncating the file to new size and then writing the data.

Now, kafs has a lock (vnode->io_lock) that serves to serialise
operations against a specific vnode (ie.  inode), but the parameters for
the op are set before the lock is taken.  This allows two writebacks
(say sync and kswapd) to race - and if writes are ongoing the writeback
for a later write could occur before the writeback for an earlier one if
the latter gets interrupted.

Note that afs_writepages() cannot take i_mutex and only takes a shared
lock on vnode->validate_lock.

Also note that the server does the truncation and the write inside a
lock, so there's no problem at that end.

Fix this by moving the calculation for the proposed new i_size inside
the vnode->io_lock.  Also reset the iterator (which we might have read
from) and update the mtime setting there.

Fixes: bd80d8a80e12 ("afs: Use ITER_XARRAY for writing")
Reported-by: Marc Dionne <marc.dionne at auristor.com>
Signed-off-by: David Howells <dhowells at redhat.com>
Reviewed-by: Jeffrey Altman <jaltman at auristor.com>
Reviewed-by: Marc Dionne <marc.dionne at auristor.com>
cc: linux-afs at lists.infradead.org
cc: linux-fsdevel at vger.kernel.org
Link: https://lore.kernel.org/r/3526895.1687960024@warthog.procyon.org.uk/
Signed-off-by: Linus Torvalds <torvalds at linux-foundation.org>
Signed-off-by: Sasha Levin <sashal at kernel.org>
---
 fs/afs/write.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/fs/afs/write.c b/fs/afs/write.c
index 8750b99c3f566..c1f4391ccd7c6 100644
--- a/fs/afs/write.c
+++ b/fs/afs/write.c
@@ -413,17 +413,19 @@ static int afs_store_data(struct afs_vnode *vnode, struct iov_iter *iter, loff_t
 	afs_op_set_vnode(op, 0, vnode);
 	op->file[0].dv_delta = 1;
 	op->file[0].modification = true;
-	op->store.write_iter = iter;
 	op->store.pos = pos;
 	op->store.size = size;
-	op->store.i_size = max(pos + size, vnode->netfs.remote_i_size);
 	op->store.laundering = laundering;
-	op->mtime = vnode->netfs.inode.i_mtime;
 	op->flags |= AFS_OPERATION_UNINTR;
 	op->ops = &afs_store_data_operation;
 
 try_next_key:
 	afs_begin_vnode_operation(op);
+
+	op->store.write_iter = iter;
+	op->store.i_size = max(pos + size, vnode->netfs.remote_i_size);
+	op->mtime = vnode->netfs.inode.i_mtime;
+
 	afs_wait_for_operation(op);
 
 	switch (op->error) {
-- 
2.39.2






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