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Wed, 09 Oct 2024 04:15:27 -0700 (PDT) Message-ID: <99089b03-e9f0-4fe8-94b2-11e75f9e5243@suse.com> Date: Wed, 9 Oct 2024 13:15:26 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] tools/xg: increase LZMA_BLOCK_SIZE for uncompressing the kernel To: Andrew Cooper Cc: Anthony PERARD , xen-devel@lists.xenproject.org, Juergen Gross , =?UTF-8?Q?Marek_Marczykowski-G=C3=B3recki?= References: <20241008213225.728922-1-marmarek@invisiblethingslab.com> <226bc9b3-6741-4cb6-917b-1164e340a19d@suse.com> <00ac0665-e4ff-4a56-8537-e81573710c3c@suse.com> Content-Language: en-US From: Jan Beulich Autocrypt: addr=jbeulich@suse.com; keydata= xsDiBFk3nEQRBADAEaSw6zC/EJkiwGPXbWtPxl2xCdSoeepS07jW8UgcHNurfHvUzogEq5xk hu507c3BarVjyWCJOylMNR98Yd8VqD9UfmX0Hb8/BrA+Hl6/DB/eqGptrf4BSRwcZQM32aZK 7Pj2XbGWIUrZrd70x1eAP9QE3P79Y2oLrsCgbZJfEwCgvz9JjGmQqQkRiTVzlZVCJYcyGGsD /0tbFCzD2h20ahe8rC1gbb3K3qk+LpBtvjBu1RY9drYk0NymiGbJWZgab6t1jM7sk2vuf0Py O9Hf9XBmK0uE9IgMaiCpc32XV9oASz6UJebwkX+zF2jG5I1BfnO9g7KlotcA/v5ClMjgo6Gl MDY4HxoSRu3i1cqqSDtVlt+AOVBJBACrZcnHAUSuCXBPy0jOlBhxPqRWv6ND4c9PH1xjQ3NP nxJuMBS8rnNg22uyfAgmBKNLpLgAGVRMZGaGoJObGf72s6TeIqKJo/LtggAS9qAUiuKVnygo 3wjfkS9A3DRO+SpU7JqWdsveeIQyeyEJ/8PTowmSQLakF+3fote9ybzd880fSmFuIEJldWxp Y2ggPGpiZXVsaWNoQHN1c2UuY29tPsJgBBMRAgAgBQJZN5xEAhsDBgsJCAcDAgQVAggDBBYC AwECHgECF4AACgkQoDSui/t3IH4J+wCfQ5jHdEjCRHj23O/5ttg9r9OIruwAn3103WUITZee e7Sbg12UgcQ5lv7SzsFNBFk3nEQQCACCuTjCjFOUdi5Nm244F+78kLghRcin/awv+IrTcIWF hUpSs1Y91iQQ7KItirz5uwCPlwejSJDQJLIS+QtJHaXDXeV6NI0Uef1hP20+y8qydDiVkv6l IreXjTb7DvksRgJNvCkWtYnlS3mYvQ9NzS9PhyALWbXnH6sIJd2O9lKS1Mrfq+y0IXCP10eS FFGg+Av3IQeFatkJAyju0PPthyTqxSI4lZYuJVPknzgaeuJv/2NccrPvmeDg6Coe7ZIeQ8Yj t0ARxu2xytAkkLCel1Lz1WLmwLstV30g80nkgZf/wr+/BXJW/oIvRlonUkxv+IbBM3dX2OV8 AmRv1ySWPTP7AAMFB/9PQK/VtlNUJvg8GXj9ootzrteGfVZVVT4XBJkfwBcpC/XcPzldjv+3 HYudvpdNK3lLujXeA5fLOH+Z/G9WBc5pFVSMocI71I8bT8lIAzreg0WvkWg5V2WZsUMlnDL9 mpwIGFhlbM3gfDMs7MPMu8YQRFVdUvtSpaAs8OFfGQ0ia3LGZcjA6Ik2+xcqscEJzNH+qh8V m5jjp28yZgaqTaRbg3M/+MTbMpicpZuqF4rnB0AQD12/3BNWDR6bmh+EkYSMcEIpQmBM51qM EKYTQGybRCjpnKHGOxG0rfFY1085mBDZCH5Kx0cl0HVJuQKC+dV2ZY5AqjcKwAxpE75MLFkr wkkEGBECAAkFAlk3nEQCGwwACgkQoDSui/t3IH7nnwCfcJWUDUFKdCsBH/E5d+0ZnMQi+G0A nAuWpQkjM1ASeQwSHEeAWPgskBQL In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit On 09.10.2024 13:08, Andrew Cooper wrote: > On 09/10/2024 11:26 am, Juergen Gross wrote: >> On 09.10.24 12:19, Jan Beulich wrote: >>> On 09.10.2024 11:52, Marek Marczykowski-Górecki wrote: >>>> On Wed, Oct 09, 2024 at 09:19:57AM +0200, Jan Beulich wrote: >>>>> On 08.10.2024 23:32, Marek Marczykowski-Górecki wrote: >>>>>> --- a/tools/libs/guest/xg_dom_bzimageloader.c >>>>>> +++ b/tools/libs/guest/xg_dom_bzimageloader.c >>>>>> @@ -272,8 +272,7 @@ static int _xc_try_lzma_decode( >>>>>>       return retval; >>>>>>   } >>>>>>   -/* 128 Mb is the minimum size (half-way) documented to work for >>>>>> all inputs. */ >>>>>> -#define LZMA_BLOCK_SIZE (128*1024*1024) >>>>>> +#define LZMA_BLOCK_SIZE (256*1024*1024) >>>>> >>>>> That's as arbitrary as before, now just not even with a comment at >>>>> least >>>>> hinting at it being arbitrary. Quoting from one of the LZMA API >>>>> headers: >>>>> >>>>>      * Decoder already supports dictionaries up to 4 GiB - 1 B (i.e. >>>>>      * UINT32_MAX), so increasing the maximum dictionary size of the >>>>>      * encoder won't cause problems for old decoders. >>>>> >>>>> IOW - what if the Linux folks decided to increase the dictionary size >>>>> further? I therefore wonder whether we don't need to make this more >>>>> dynamic, perhaps by peeking into the header to obtain the dictionary >>>>> size used. The one thing I'm not sure about is whether there can't be >>>>> multiple such headers throughout the file, and hence (in principle) >>>>> differing dictionary sizes. >>>> >>>> What is the purpose of this block size limit? From the error >>>> message, it >>>> seems to be avoiding excessive memory usage during decompression (which >>>> could be DoS via OOM). If that's the case, then taking the limit from >>>> the kernel binary itself will miss this point (especially in case of >>>> pygrub or similar, but there may be other cases of not-fully-trusted >>>> kernel binaries). >>> >>> Indeed. The question then simply is: Where do we want to draw the line >>> between what we permit and what we reject? >> >> IMHO the most natural solution would be to use guest memory for this >> purpose. >> OTOH this probably would require a significant rework of libxenguest. > > That was XSA-25.  There are toolstack-provided limits on kernel&initrd > sizes. Which probably can't be directly applied to dictionary size used during (de)compression. Jan