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maple_tree: disable mas_wr_append() when other readers are possible 

commit cfeb6ae8bcb96ccf674724f223661bbcef7b0d0b
Author: Liam R. Howlett <Liam.Howlett@oracle.com>
Date:   Fri Aug 18 20:43:55 2023 -0400

    maple_tree: disable mas_wr_append() when other readers are possible

    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@oracle.com
    Fixes: 54a611b60590 ("Maple Tree: add new data structure")
    Signed-off-by: Liam R. Howlett <Liam.Howlett@oracle.com>
    Cc: <stable@vger.kernel.org>
    Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

JIRA: https://issues.redhat.com/browse/RHEL-5595
Signed-off-by: Nico Pache <npache@redhat.com>
This commit is contained in:
Nico Pache 2023-09-26 10:23:02 -06:00
parent 6995d5aa65
commit 9d911ab8ce
1 changed files with 7 additions and 0 deletions
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7
lib/maple_tree.c
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@ -4265,6 +4265,10 @@ static inline unsigned char mas_wr_new_end(struct ma_wr_state *wr_mas)
* mas_wr_append: Attempt to append
* @wr_mas: the maple write state
*
* This is currently unsafe in rcu mode since the end of the node may be cached
* by readers while the node contents may be updated which could result in
* inaccurate information.
*
* Return: True if appended, false otherwise
*/
static inline bool mas_wr_append(struct ma_wr_state *wr_mas)
@ -4274,6 +4278,9 @@ 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->offset != wr_mas->node_end)
return false;