Re: [PATCH v3] erofs: cap LZMA stream pool size

[Gao Xiang <hsiangkao@linux.alibaba.com>]

Hi Machael,

On 2026/7/17 11:43, Gao Xiang wrote:
> 
> 
> On 2026/7/14 19:47, Michael Bommarito wrote:
>> 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>
> 
I submitted the following version to -next:

 From 4ec57610a769cd93027d12134c75160390b23b08 Mon Sep 17 00:00:00 2001
From: Michael Bommarito <michael.bommarito@gmail.com>
Date: Tue, 14 Jul 2026 07:47:29 -0400
Subject: 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>
---
  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..36f027c1c5ac5 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 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 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);
--