Patch "afs: Fix accidental truncation when storing data" has been added to the 6.4-stable tree

[Sasha Levin <sashal@xxxxxxxxxx>]

This is a note to let you know that I've just added the patch titled

    afs: Fix accidental truncation when storing data

to the 6.4-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     afs-fix-accidental-truncation-when-storing-data.patch
and it can be found in the queue-6.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.



commit 8b9acf5280d10e836fb3ea6be279353858dc4fdb
Author: David Howells <dhowells@xxxxxxxxxx>
Date:   Tue Jul 4 20:22:15 2023 +0100

    afs: Fix accidental truncation when storing data
    
    [ 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@xxxxxxxxxxxx>
    Signed-off-by: David Howells <dhowells@xxxxxxxxxx>
    Reviewed-by: Jeffrey Altman <jaltman@xxxxxxxxxxxx>
    Reviewed-by: Marc Dionne <marc.dionne@xxxxxxxxxxxx>
    cc: linux-afs@xxxxxxxxxxxxxxxxxxx
    cc: linux-fsdevel@xxxxxxxxxxxxxxx
    Link: https://lore.kernel.org/r/3526895.1687960024@xxxxxxxxxxxxxxxxxxxxxx/
    Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
    Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>

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) {