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![]() Modify the request_module to prefix the file system type with "fs-" and add aliases to all of the filesystems that can be built as modules to match. A common practice is to build all of the kernel code and leave code that is not commonly needed as modules, with the result that many users are exposed to any bug anywhere in the kernel. Looking for filesystems with a fs- prefix limits the pool of possible modules that can be loaded by mount to just filesystems trivially making things safer with no real cost. Using aliases means user space can control the policy of which filesystem modules are auto-loaded by editing /etc/modprobe.d/*.conf with blacklist and alias directives. Allowing simple, safe, well understood work-arounds to known problematic software. This also addresses a rare but unfortunate problem where the filesystem name is not the same as it's module name and module auto-loading would not work. While writing this patch I saw a handful of such cases. The most significant being autofs that lives in the module autofs4. This is relevant to user namespaces because we can reach the request module in get_fs_type() without having any special permissions, and people get uncomfortable when a user specified string (in this case the filesystem type) goes all of the way to request_module. After having looked at this issue I don't think there is any particular reason to perform any filtering or permission checks beyond making it clear in the module request that we want a filesystem module. The common pattern in the kernel is to call request_module() without regards to the users permissions. In general all a filesystem module does once loaded is call register_filesystem() and go to sleep. Which means there is not much attack surface exposed by loading a filesytem module unless the filesystem is mounted. In a user namespace filesystems are not mounted unless .fs_flags = FS_USERNS_MOUNT, which most filesystems do not set today. Acked-by: Serge Hallyn <serge.hallyn@canonical.com> Acked-by: Kees Cook <keescook@chromium.org> Reported-by: Kees Cook <keescook@google.com> Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com> |
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uncompress.c |
Notes on Filesystem Layout -------------------------- These notes describe what mkcramfs generates. Kernel requirements are a bit looser, e.g. it doesn't care if the <file_data> items are swapped around (though it does care that directory entries (inodes) in a given directory are contiguous, as this is used by readdir). All data is currently in host-endian format; neither mkcramfs nor the kernel ever do swabbing. (See section `Block Size' below.) <filesystem>: <superblock> <directory_structure> <data> <superblock>: struct cramfs_super (see cramfs_fs.h). <directory_structure>: For each file: struct cramfs_inode (see cramfs_fs.h). Filename. Not generally null-terminated, but it is null-padded to a multiple of 4 bytes. The order of inode traversal is described as "width-first" (not to be confused with breadth-first); i.e. like depth-first but listing all of a directory's entries before recursing down its subdirectories: the same order as `ls -AUR' (but without the /^\..*:$/ directory header lines); put another way, the same order as `find -type d -exec ls -AU1 {} \;'. Beginning in 2.4.7, directory entries are sorted. This optimization allows cramfs_lookup to return more quickly when a filename does not exist, speeds up user-space directory sorts, etc. <data>: One <file_data> for each file that's either a symlink or a regular file of non-zero st_size. <file_data>: nblocks * <block_pointer> (where nblocks = (st_size - 1) / blksize + 1) nblocks * <block> padding to multiple of 4 bytes The i'th <block_pointer> for a file stores the byte offset of the *end* of the i'th <block> (i.e. one past the last byte, which is the same as the start of the (i+1)'th <block> if there is one). The first <block> immediately follows the last <block_pointer> for the file. <block_pointer>s are each 32 bits long. The order of <file_data>'s is a depth-first descent of the directory tree, i.e. the same order as `find -size +0 \( -type f -o -type l \) -print'. <block>: The i'th <block> is the output of zlib's compress function applied to the i'th blksize-sized chunk of the input data. (For the last <block> of the file, the input may of course be smaller.) Each <block> may be a different size. (See <block_pointer> above.) <block>s are merely byte-aligned, not generally u32-aligned. Holes ----- This kernel supports cramfs holes (i.e. [efficient representation of] blocks in uncompressed data consisting entirely of NUL bytes), but by default mkcramfs doesn't test for & create holes, since cramfs in kernels up to at least 2.3.39 didn't support holes. Run mkcramfs with -z if you want it to create files that can have holes in them. Tools ----- The cramfs user-space tools, including mkcramfs and cramfsck, are located at <http://sourceforge.net/projects/cramfs/>. Future Development ================== Block Size ---------- (Block size in cramfs refers to the size of input data that is compressed at a time. It's intended to be somewhere around PAGE_CACHE_SIZE for cramfs_readpage's convenience.) The superblock ought to indicate the block size that the fs was written for, since comments in <linux/pagemap.h> indicate that PAGE_CACHE_SIZE may grow in future (if I interpret the comment correctly). Currently, mkcramfs #define's PAGE_CACHE_SIZE as 4096 and uses that for blksize, whereas Linux-2.3.39 uses its PAGE_CACHE_SIZE, which in turn is defined as PAGE_SIZE (which can be as large as 32KB on arm). This discrepancy is a bug, though it's not clear which should be changed. One option is to change mkcramfs to take its PAGE_CACHE_SIZE from <asm/page.h>. Personally I don't like this option, but it does require the least amount of change: just change `#define PAGE_CACHE_SIZE (4096)' to `#include <asm/page.h>'. The disadvantage is that the generated cramfs cannot always be shared between different kernels, not even necessarily kernels of the same architecture if PAGE_CACHE_SIZE is subject to change between kernel versions (currently possible with arm and ia64). The remaining options try to make cramfs more sharable. One part of that is addressing endianness. The two options here are `always use little-endian' (like ext2fs) or `writer chooses endianness; kernel adapts at runtime'. Little-endian wins because of code simplicity and little CPU overhead even on big-endian machines. The cost of swabbing is changing the code to use the le32_to_cpu etc. macros as used by ext2fs. We don't need to swab the compressed data, only the superblock, inodes and block pointers. The other part of making cramfs more sharable is choosing a block size. The options are: 1. Always 4096 bytes. 2. Writer chooses blocksize; kernel adapts but rejects blocksize > PAGE_CACHE_SIZE. 3. Writer chooses blocksize; kernel adapts even to blocksize > PAGE_CACHE_SIZE. It's easy enough to change the kernel to use a smaller value than PAGE_CACHE_SIZE: just make cramfs_readpage read multiple blocks. The cost of option 1 is that kernels with a larger PAGE_CACHE_SIZE value don't get as good compression as they can. The cost of option 2 relative to option 1 is that the code uses variables instead of #define'd constants. The gain is that people with kernels having larger PAGE_CACHE_SIZE can make use of that if they don't mind their cramfs being inaccessible to kernels with smaller PAGE_CACHE_SIZE values. Option 3 is easy to implement if we don't mind being CPU-inefficient: e.g. get readpage to decompress to a buffer of size MAX_BLKSIZE (which must be no larger than 32KB) and discard what it doesn't need. Getting readpage to read into all the covered pages is harder. The main advantage of option 3 over 1, 2, is better compression. The cost is greater complexity. Probably not worth it, but I hope someone will disagree. (If it is implemented, then I'll re-use that code in e2compr.) Another cost of 2 and 3 over 1 is making mkcramfs use a different block size, but that just means adding and parsing a -b option. Inode Size ---------- Given that cramfs will probably be used for CDs etc. as well as just silicon ROMs, it might make sense to expand the inode a little from its current 12 bytes. Inodes other than the root inode are followed by filename, so the expansion doesn't even have to be a multiple of 4 bytes.