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We recently had an oops reported on a 4.14 kernel in xfs_reclaim_inodes_count() where sb->s_fs_info pointed to garbage and so the m_perag_tree lookup walked into lala land. It produces an oops down this path during the failed mount: radix_tree_gang_lookup_tag+0xc4/0x130 xfs_perag_get_tag+0x37/0xf0 xfs_reclaim_inodes_count+0x32/0x40 xfs_fs_nr_cached_objects+0x11/0x20 super_cache_count+0x35/0xc0 shrink_slab.part.66+0xb1/0x370 shrink_node+0x7e/0x1a0 try_to_free_pages+0x199/0x470 __alloc_pages_slowpath+0x3a1/0xd20 __alloc_pages_nodemask+0x1c3/0x200 cache_grow_begin+0x20b/0x2e0 fallback_alloc+0x160/0x200 kmem_cache_alloc+0x111/0x4e0 The problem is that the superblock shrinker is running before the filesystem structures it depends on have been fully set up. i.e. the shrinker is registered in sget(), before ->fill_super() has been called, and the shrinker can call into the filesystem before fill_super() does it's setup work. Essentially we are exposed to both use-after-free and use-before-initialisation bugs here. To fix this, add a check for the SB_BORN flag in super_cache_count. In general, this flag is not set until ->fs_mount() completes successfully, so we know that it is set after the filesystem setup has completed. This matches the trylock_super() behaviour which will not let super_cache_scan() run if SB_BORN is not set, and hence will not allow the superblock shrinker from entering the filesystem while it is being set up or after it has failed setup and is being torn down. Cc: stable@kernel.org Signed-Off-By: Dave Chinner <dchinner@redhat.com> Signed-off-by: Al Viro <viro@zeniv.linux.org.uk> |
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Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
See Documentation/00-INDEX for a list of what is contained in each file.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.