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-
-Using liblzma securely
-----------------------
-
-0. Introduction
-
- This document discusses how to use liblzma securely. There are issues
- that don't apply to zlib or libbzip2, so reading this document is
- strongly recommended even for those who are very familiar with zlib
- or libbzip2.
-
- While making liblzma itself as secure as possible is essential, it's
- out of scope of this document.
-
-
-1. Memory usage
-
- The memory usage of liblzma varies a lot.
-
-
-1.1. Problem sources
-
-1.1.1. Block coder
-
- The memory requirements of Block encoder depend on the used filters
- and their settings. The memory requirements of the Block decoder
- depend on the which filters and with which filter settings the Block
- was encoded. Usually the memory requirements of a decoder are equal
- or less than the requirements of the encoder with the same settings.
-
- While the typical memory requirements to decode a Block is from a few
- hundred kilobytes to tens of megabytes, a maliciously constructed
- files can require a lot more RAM to decode. With the current filters,
- the maximum amount is about 7 GiB. If you use multi-threaded decoding,
- every Block can require this amount of RAM, thus a four-threaded
- decoder could suddenly try to allocate 28 GiB of RAM.
-
- If you don't limit the maximum memory usage in any way, and there are
- no resource limits set on the operating system side, one malicious
- input file can run the system out of memory, or at least make it swap
- badly for a long time. This is exceptionally bad on servers e.g.
- email server doing virus scanning on incoming messages.
-
-
-1.1.2. Metadata decoder
-
- Multi-Block .lzma files contain at least one Metadata Block.
- Externally the Metadata Blocks are similar to Data Blocks, so all
- the issues mentioned about memory usage of Data Blocks applies to
- Metadata Blocks too.
-
- The uncompressed content of Metadata Blocks contain information about
- the Stream as a whole, and optionally some Extra Records. The
- information about the Stream is kept in liblzma's internal data
- structures in RAM. Extra Records can contain arbitrary data. They are
- not interpreted by liblzma, but liblzma will provide them to the
- application in uninterpreted form if the application wishes so.
-
- Usually the Uncompressed Size of a Metadata Block is small. Even on
- extreme cases, it shouldn't be much bigger than a few megabytes. Once
- the Metadata has been parsed into native data structures in liblzma,
- it usually takes a little more memory than in the encoded form. For
- all normal files, this is no problem, since the resulting memory usage
- won't be too much.
-
- The problem is that a maliciously constructed Metadata Block can
- contain huge amount of "information", which liblzma will try to store
- in its internal data structures. This may cause liblzma to allocate
- all the available RAM unless some kind of resource usage limits are
- applied.
-
- Note that the Extra Records in Metadata are always parsed but, but
- memory is allocated for them only if the application has requested
- liblzma to provide the Extra Records to the application.
-
-
-1.2. Solutions
-
- If you need to decode files from untrusted sources (most people do),
- you must limit the memory usage to avoid denial of service (DoS)
- conditions caused by malicious input files.
-
- The first step is to find out how much memory you are allowed consume
- at maximum. This may be a hardcoded constant or derived from the
- available RAM; whatever is appropriate in the application.
-
- The simplest solution is to use setrlimit() if the kernel supports
- RLIMIT_AS, which limits the memory usage of the whole process.
- For more portable and fine-grained limiting, you can use
- memory limiter functions found from <lzma/memlimit.h>.
-
-
-1.2.1. Encoder
-
- lzma_memory_usage() will give you a rough estimate about the memory
- usage of the given filter chain. To dramatically simplify the internal
- implementation, this function doesn't take into account all the small
- helper data structures needed in various places; only the structures
- with significant memory usage are taken into account. Still, the
- accuracy of this function should be well within a mebibyte.
-
- The Subblock filter is a special case. If a Subfilter has been
- specified, it isn't taken into account when lzma_memory_usage()
- calculates the memory usage. You need to calculate the memory usage
- of the Subfilter separately.
-
- Keeping track of Blocks in a Multi-Block Stream takes a few dozen
- bytes of RAM per Block (size of the lzma_index structure plus overhead
- of malloc()). It isn't a good idea to put tens of thousands of Blocks
- into a Stream unless you have a very good reason to do so (compressed
- dictionary could be an example of such situation).
-
- Also keep the number and sizes of Extra Records sane. If you produce
- the list of Extra Records automatically from some untrusted source,
- you should not only validate the content of these Records, but also
- their memory usage.
-
-
-1.2.2. Decoder
-
- A single-threaded decoder should simply use a memory limiter and
- indicate an error if it runs out of memory.
-
- Memory-limiting with multi-threaded decoding is tricky. The simple
- solution is to divide the maximum allowed memory usage with the
- maximum allowed threads, and give each Block decoder their own
- independent lzma_memory_limiter. The drawback is that if one Block
- needs notably more RAM than any other Block, the decoder will run out
- of memory when in reality there would be plenty of free RAM.
-
- An attractive alternative would be using shared lzma_memory_limiter.
- Depending on the application and the expected type of input, this may
- either be the best solution or a source of hard-to-repeat problems.
- Consider the following requirements:
- - You use a maximum of n threads.
- - x(i) is the decoder memory requirements of the Block number i
- in an expected input Stream.
- - The memory limiter is set to higher value than the sum of n
- highest values x(i).
-
- (If you are better at explaining the above conditions, please
- contribute your improved version.)
-
- If the above conditions aren't met, it is possible that the decoding
- will fail unpredictably. That is, on the same machine using the same
- settings, the decoding may sometimes succeed and sometimes fail. This
- is because sometimes threads may run so that the Blocks with highest
- memory usage are tried to be decoded at the same time.
-
- Most .lzma files have all the Blocks encoded with identical settings,
- or at least the memory usage won't vary dramatically. That's why most
- multi-threaded decoders probably want to use the simple "separate
- lzma_memory_limiter for each thread" solution, possibly falling back
- to single-threaded mode in case the per-thread memory limits aren't
- enough in multi-threaded mode.
-
-FIXME: Memory usage of Stream info.
-
-[
-
-]
-
-
-2. Huge uncompressed output
-
-2.1. Data Blocks
-
- Decoding a tiny .lzma file can produce huge amount of uncompressed
- output. There is an example file of 45 bytes, which decodes to 64 PiB
- (that's 2^56 bytes). Uncompressing such a file to disk is likely to
- fill even a bigger disk array. If the data is written to a pipe, it
- may not fill the disk, but would still take very long time to finish.
-
- To avoid denial of service conditions caused by huge amount of
- uncompressed output, applications using liblzma should use some method
- to limit the amount of output produced. The exact method depends on
- the application.
-
- All valid .lzma Streams make it possible to find out the uncompressed
- size of the Stream without actually uncompressing the data. This
- information is available in at least one of the Metadata Blocks.
- Once the uncompressed size is parsed, the decoder can verify that
- it doesn't exceed certain limits (e.g. available disk space).
-
- When the uncompressed size is known, the decoder can actively keep
- track of the amount of output produced so far, and that it doesn't
- exceed the known uncompressed size. If it does exceed, the file is
- known to be corrupt and an error should be indicated without
- continuing to decode the rest of the file.
-
- Unfortunately, finding the uncompressed size beforehand is often
- possible only in non-streamed mode, because the needed information
- could be in the Footer Metdata Block, which (obviously) is at the
- end of the Stream. In purely streamed mode decoding, one may need to
- use some rough arbitrary limits to prevent the problems described in
- the beginning of this section.
-
-
-2.2. Metadata
-
- Metadata is stored in Metadata Blocks, which are very similar to
- Data Blocks. Thus, the uncompressed size can be huge just like with
- Data Blocks. The difference is, that the contents of Metadata Blocks
- aren't given to the application as is, but parsed by liblzma. Still,
- reading through a huge Metadata can take very long time, effectively
- creating a denial of service like piping decoded a Data Block to
- another process would do.
-
- At first it would seem that using a memory limiter would prevent
- this issue as a side effect. But it does so only if the application
- requests liblzma to allocate the Extra Records and provide them to
- the application. If Extra Records aren't requested, they aren't
- allocated either. Still, the Extra Records are being read through
- to validate that the Metadata is in proper format.
-
- The solution is to limit the Uncompressed Size of a Metadata Block
- to some relatively large value. This will make liblzma to give an
- error when the given limit is reached.
-