Imported to git.

1 files changed, 219 insertions, 0 deletions

diff --git a/doc/liblzma-security.txt b/doc/liblzma-security.txt
new file mode 100644
index 00000000..487637ed
--- /dev/null
+++ b/doc/liblzma-security.txt
@@ -0,0 +1,219 @@
+
+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 limitting, you can use
+    memory limitter 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 limitter and
+    indicate an error if it runs out of memory.
+
+    Memory-limitting 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_limitter. 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_limitter.
+    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 at maximum of n threads.
+      - x(i) is the decoder memory requirements of the Block number i
+        in an expected input Stream.
+      - The memory limitter 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_limitter for each thread" solution, possibly fallbacking
+    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 limitter 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.
+