ext4: fix spectre gadget in ext4_mb_regular_allocator()

'ac->ac_g_ex.fe_len' is a user-controlled value which is used in the
derivation of 'ac->ac_2order'. 'ac->ac_2order', in turn, is used to
index arrays which makes it a potential spectre gadget. Fix this by
sanitizing the value assigned to 'ac->ac2_order'.  This covers the
following accesses found with the help of smatch:

* fs/ext4/mballoc.c:1896 ext4_mb_simple_scan_group() warn: potential
  spectre issue 'grp->bb_counters' [w] (local cap)

* fs/ext4/mballoc.c:445 mb_find_buddy() warn: potential spectre issue
  'EXT4_SB(e4b->bd_sb)->s_mb_offsets' [r] (local cap)

* fs/ext4/mballoc.c:446 mb_find_buddy() warn: potential spectre issue
  'EXT4_SB(e4b->bd_sb)->s_mb_maxs' [r] (local cap)

Suggested-by: Josh Poimboeuf <jpoimboe@redhat.com>
Signed-off-by: Jeremy Cline <jcline@redhat.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Cc: stable@vger.kernel.org
CVE-2017-5753

(cherry picked from commit 1a5d5e5d51e75a5bca67dadbcea8c841934b7b85)
Signed-off-by: Juerg Haefliger <juergh@canonical.com>
Acked-by: Stefan Bader <stefan.bader@canonical.com>
Acked-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>

diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
index b2a73060bff850875a4a1ba4205a1e11543bb01f..1413e9513b615f9e0c692a8b21069ba93cdff137 100644 (file)
--- a/fs/ext4/mballoc.c
+++ b/fs/ext4/mballoc.c
@@ -26,6 +26,7 @@
 #include <linux/log2.h>
 #include <linux/module.h>
 #include <linux/slab.h>
 #include <linux/log2.h>
 #include <linux/module.h>
 #include <linux/slab.h>

+#include <linux/nospec.h>

 #include <linux/backing-dev.h>
 #include <trace/events/ext4.h>
 
 #include <linux/backing-dev.h>
 #include <trace/events/ext4.h>
 


@@ -2152,7 +2153,8 @@ ext4_mb_regular_allocator(struct ext4_allocation_context *ac)
                 * This should tell if fe_len is exactly power of 2
                 */
                if ((ac->ac_g_ex.fe_len & (~(1 << (i - 1)))) == 0)
                 * This should tell if fe_len is exactly power of 2
                 */
                if ((ac->ac_g_ex.fe_len & (~(1 << (i - 1)))) == 0)

-                       ac->ac_2order = i - 1;
+                       ac->ac_2order = array_index_nospec(i - 1,
+                                                          sb->s_blocksize_bits + 2);

        }
 
        /* if stream allocation is enabled, use global goal */
        }
 
        /* if stream allocation is enabled, use global goal */