[PATCH v3] erofs: cap LZMA stream pool size

[Michael Bommarito <michael.bommarito@gmail.com>]

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 attacker-supplied EROFS image mounted by the system can pin up
to 8 MiB times the LZMA stream count of kernel vmalloc memory.

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 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>
---
v3: rename the Kconfig option to EROFS_FS_ZIP_LZMA_DEFAULT_MAX_STREAMS
    and only cap the default (num_possible_cpus); an explicit non-zero
    lzma_streams module parameter is now honoured unchanged.  Simplified
    the Kconfig help text and dropped the in-code comment, per Gao
    Xiang's review.
v2: https://lore.kernel.org/linux-erofs/20260711143419.2762894-1-michael.bommarito@gmail.com/

Evidence: the stock code sets the stream count to num_possible_cpus() when
lzma_streams is unset, and z_erofs_load_lzma_config() then preallocates one
image-supplied dictionary (up to Z_EROFS_LZMA_MAX_DICT_SIZE, 8 MiB) per
stream, so on a host with many CPUs a single small mounted image reserves
num_possible_cpus() x up-to-8 MiB of vmalloc decoder state until the module
is unloaded.  With this patch an unset lzma_streams caps the default at
CONFIG_EROFS_FS_ZIP_LZMA_DEFAULT_MAX_STREAMS (16, i.e. 128 MiB worst case),
while an explicit non-zero lzma_streams= is left unbounded.  Built with W=1,
no new warnings; boots and mounts an LZMA image with the capped default and
with lzma_streams= overriding it.

 fs/erofs/Kconfig             | 14 ++++++++++++++
 fs/erofs/decompressor_lzma.c |  3 ++-
 2 files changed, 16 insertions(+), 1 deletion(-)

diff --git a/fs/erofs/Kconfig b/fs/erofs/Kconfig
index 4789b1077d8ce..8948cb6314e07 100644
--- 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 1024
+	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 f6692d0f2f04d..6b0cdb446c6ad 100644
--- 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);
-- 
2.53.0