[PATCH v2] module: decompress: check return value of module_extend_max_pages()

[Andrii Kuchmenko <capyenglishlite@gmail.com>]

module_extend_max_pages() calls kvrealloc() internally and returns
-ENOMEM on allocation failure. The return value is never checked.
The decompression loop then continues calling module_get_next_page(),
which writes struct page pointers into info->pages[]. When used_pages
reaches the stale max_pages value (not updated due to the failed
extend), a subsequent write to info->pages[used_pages++] goes out of
bounds into adjacent heap memory.

Adjacent slab objects in the same kmalloc cache (pipe_buffer,
seq_operations, cred) can be corrupted, potentially leading to local
privilege escalation on kernels without SLAB_VIRTUAL mitigation.

The call order in finit_module() is:

  module_decompress()    <- vulnerable, runs FIRST
  load_module()
    module_sig_check()   <- signature check, runs SECOND

Decompression happens before signature verification. A crafted
compressed module submitted via finit_module(MODULE_INIT_COMPRESSED_FILE)
reaches this code path before any signature gate is applied. On kernels
with module.sig_enforce=0 (default without SecureBoot) or with
unprivileged user namespaces (Ubuntu, Debian default), this is
reachable without CAP_SYS_MODULE.

Confirmed present in mainline (tested on v6.14-rc3).

Fix: add the missing error check after module_extend_max_pages() and
return immediately on failure. This matches the pattern used by every
other kvrealloc() caller in the module loading path.

Fixes: 169a58ad824d ("module: add in-kernel support for decompressing")
Cc: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Cc: Luis Chamberlain <mcgrof@kernel.org>
Cc: stable@vger.kernel.org
Signed-off-by: Andrii Kuchmenko <capyenglishlite@gmail.com>
---
Changes in v2:
 - Remove unnecessary initialization of 'error' to 0 (Christophe Leroy)
 - Remove unrelated blank line after if (error) return error (Christophe Leroy)

 kernel/module/decompress.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/kernel/module/decompress.c b/kernel/module/decompress.c
--- a/kernel/module/decompress.c
+++ b/kernel/module/decompress.c
@@ -XXX,9 +XXX,12 @@ int module_decompress(struct load_info *info,
 				const void *buf, size_t size)
 {
 	unsigned int n_pages;
 	int error;
 	ssize_t data_size;
 
 	n_pages = DIV_ROUND_UP(size, PAGE_SIZE) * 2;
 	error = module_extend_max_pages(info, n_pages);
+	if (error)
+		return error;
 	data_size = MODULE_DECOMPRESS_FN(info, buf, size);
 	if (data_size < 0) {
 		error = data_size;
-- 
2.39.0