erofs: cap LZMA stream pool size

fs/erofs/decompressor_lzma.c sizes the module-global MicroLZMA stream
pool from num_possible_cpus() when the lzma_streams module parameter is
unset, then z_erofs_load_lzma_config() preallocates one image-supplied
dictionary per stream, accepting dictionaries up to 8 MiB.  On high-CPU
systems, a small EROFS image can pin hundreds of MiB of vmalloc-backed
decoder state until the erofs module is unloaded.

Impact: An EROFS image mounted by the system can pin up to 8 MiB of
vmalloc memory per LZMA stream, either as intended or unexpectedly.

Bound the default stream count by a new
CONFIG_EROFS_FS_ZIP_LZMA_DEFAULT_MAX_STREAMS option, default 16, so the
worst-case default preallocation is 128 MiB if the number of CPUs is no
less than 16 while preserving the existing per-image dictionary limit.
An explicit lzma_streams module parameter is still honoured as-is, so
administrators who deliberately size the pool are not affected.

Fixes: 622ceaddb764 ("erofs: lzma compression support")
Cc: stable@vger.kernel.org
Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Michael Bommarito <michael.bommarito@gmail.com>
Reviewed-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>

diff --git a/fs/erofs/Kconfig b/fs/erofs/Kconfig
index 1feb28cfe557d62ad581fba466b18e0266dd415a..8ca1767dafb618ff3e977f6d2a8344ca1405b46f 100644 (file)
--- a/fs/erofs/Kconfig
+++ b/fs/erofs/Kconfig
@@ -131,6 +131,20 @@ config EROFS_FS_ZIP_LZMA
 
          Say N if you want to disable LZMA compression support.
 
+config EROFS_FS_ZIP_LZMA_DEFAULT_MAX_STREAMS
+       int "EROFS LZMA default maximum decompression streams"
+       depends on EROFS_FS_ZIP_LZMA
+       range 1 NR_CPUS
+       default 16
+       help
+         By default EROFS allocates one LZMA decompression stream per CPU.
+         Each stream can hold a dictionary of up to 8 MiB taken from the
+         mounted image, so on systems with many CPUs this can reserve a lot
+         of memory.  This caps the default; the lzma_streams module parameter
+         still overrides it.
+
+         If unsure, keep the default of 16.
+
 config EROFS_FS_ZIP_DEFLATE
        bool "EROFS DEFLATE compressed data support"
        depends on EROFS_FS_ZIP

diff --git a/fs/erofs/decompressor_lzma.c b/fs/erofs/decompressor_lzma.c
index f6692d0f2f04dbc35011497d4174ad566391f92a..6b0cdb446c6ad23e59c493c1834e095c126187d2 100644 (file)
--- a/fs/erofs/decompressor_lzma.c
+++ b/fs/erofs/decompressor_lzma.c
@@ -51,7 +51,8 @@ static int __init z_erofs_lzma_init(void)
 
        /* by default, use # of possible CPUs instead */
        if (!z_erofs_lzma_nstrms)
-               z_erofs_lzma_nstrms = num_possible_cpus();
+               z_erofs_lzma_nstrms = min_t(unsigned int, num_possible_cpus(),
+                               CONFIG_EROFS_FS_ZIP_LZMA_DEFAULT_MAX_STREAMS);
 
        for (i = 0; i < z_erofs_lzma_nstrms; ++i) {
                struct z_erofs_lzma *strm = kzalloc_obj(*strm);