Patch "maple_tree: disable mas_wr_append() when other readers are possible" has been added to the 6.4-stable tree

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

    maple_tree: disable mas_wr_append() when other readers are possible

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:
     maple_tree-disable-mas_wr_append-when-other-readers-.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 b91c7f43cef8824a77b803702f2695119b451470
Author: Liam R. Howlett <Liam.Howlett@xxxxxxxxxx>
Date:   Fri Aug 18 20:43:55 2023 -0400

    maple_tree: disable mas_wr_append() when other readers are possible
    
    [ Upstream commit cfeb6ae8bcb96ccf674724f223661bbcef7b0d0b ]
    
    The current implementation of append may cause duplicate data and/or
    incorrect ranges to be returned to a reader during an update.  Although
    this has not been reported or seen, disable the append write operation
    while the tree is in rcu mode out of an abundance of caution.
    
    During the analysis of the mas_next_slot() the following was
    artificially created by separating the writer and reader code:
    
    Writer:                                 reader:
    mas_wr_append
        set end pivot
        updates end metata
        Detects write to last slot
        last slot write is to start of slot
        store current contents in slot
        overwrite old end pivot
                                            mas_next_slot():
                                                    read end metadata
                                                    read old end pivot
                                                    return with incorrect range
        store new value
    
    Alternatively:
    
    Writer:                                 reader:
    mas_wr_append
        set end pivot
        updates end metata
        Detects write to last slot
        last lost write to end of slot
        store value
                                            mas_next_slot():
                                                    read end metadata
                                                    read old end pivot
                                                    read new end pivot
                                                    return with incorrect range
        set old end pivot
    
    There may be other accesses that are not safe since we are now updating
    both metadata and pointers, so disabling append if there could be rcu
    readers is the safest action.
    
    Link: https://lkml.kernel.org/r/20230819004356.1454718-2-Liam.Howlett@xxxxxxxxxx
    Fixes: 54a611b60590 ("Maple Tree: add new data structure")
    Signed-off-by: Liam R. Howlett <Liam.Howlett@xxxxxxxxxx>
    Cc: <stable@xxxxxxxxxxxxxxx>
    Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
    Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>

diff --git a/lib/maple_tree.c b/lib/maple_tree.c
index bb28a49d173c0..3315eaf93f563 100644
--- a/lib/maple_tree.c
+++ b/lib/maple_tree.c
@@ -4315,6 +4315,9 @@ static inline bool mas_wr_append(struct ma_wr_state *wr_mas)
 	struct ma_state *mas = wr_mas->mas;
 	unsigned char node_pivots = mt_pivots[wr_mas->type];
 
+	if (mt_in_rcu(mas->tree))
+		return false;
+
 	if ((mas->index != wr_mas->r_min) && (mas->last == wr_mas->r_max)) {
 		if (new_end < node_pivots)
 			wr_mas->pivots[new_end] = wr_mas->pivots[end];



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