Hi Michael,
On Wed, Apr 03, 2024 at 01:42:23PM GMT, Michael Weiß wrote:
> finit_module() supports the MODULE_INIT_COMPRESS_FILE flag since
> Linux 5.17. See commit b1ae6dc41eaaa ("module: add in-kernel support
> for decompressing")
>
> During implementation of a secure module loader in GyroidOS, we
> wanted to filter unsafe module parameters. To verify that only the
> two documented flags which are disabling sanity checks are unsafe,
> we had a look in the current kernel implementation.
>
> We discovered that this new flag MODULE_INIT_COMPRESS_FILE was added.
> Having a deeper look at the code, we also discovered that a new error
> code EOPNOTSUPP is possible within newer kernels.
>
> The inital commit only supported gzip and xz compression algorithms.
> Support for zstd was added in Linux 6.2 by commit 169a58ad824d8
> ("module/decompress: Support zstd in-kernel decompression")
>
> Signed-off-by: Michael Weiß <michael.weiss@xxxxxxxxxxxxxxxxxxx>
I've applied the patch, with some minor cosmetic changes:
<https://www.alejandro-colomar.es/src/alx/linux/man-pages/man-pages.git/commit/?h=contrib&id=852f6874f5c1f57e70fcdc6e68411435acea2369>
Have a lovely day!
Alex
> ---
>
> Changes in v2:
> - Update formatting (Alejandro)
> - Split EINVAL into two entries (Alejandro)
> - Fixed error in EINVAL, s/CONFIG_MODULE_DECOMPRESS/MODULE_INIT_COMPRESSED_FILE/
> - Fixed incomplete description for EOPNOTSUPP (Alejandro)
> - Link to v1: https://lore.kernel.org/r/20240329124137.630521-1-michael.weiss@xxxxxxxxxxxxxxxxxxx/
>
> ---
> man2/init_module.2 | 50 ++++++++++++++++++++++++++++++++++++++++++++++
> 1 file changed, 50 insertions(+)
>
> diff --git a/man2/init_module.2 b/man2/init_module.2
> index 95917a079..133d82b53 100644
> --- a/man2/init_module.2
> +++ b/man2/init_module.2
> @@ -107,6 +107,10 @@ Ignore symbol version hashes.
> .TP
> .B MODULE_INIT_IGNORE_VERMAGIC
> Ignore kernel version magic.
> +.TP
> +.BR MODULE_INIT_COMPRESSED_FILE " (since Linux 5.17)"
> +.\" commit b1ae6dc41eaaa98bb75671e0f3665bfda248c3e7
> +Use in-kernel module decompression.
> .P
> There are some safety checks built into a module to ensure that
> it matches the kernel against which it is loaded.
> @@ -136,6 +140,39 @@ If the kernel is built to permit forced loading (i.e., configured with
> then loading continues, otherwise it fails with the error
> .B ENOEXEC
> as expected for malformed modules.
> +.P
> +If the kernel was build with
> +.BR CONFIG_MODULE_DECOMPRESS ,
> +the in-kernel decompression feature can be used.
> +Userspace code can check if the kernel supports decompression by reading the
> +.I /sys/module/compression
> +attribute.
> +If the kernel supports decompression,
> +the compressed file can directly be passed to
> +.BR finit_module ()
> +using the
> +.B MODULE_INIT_COMPRESSED_FILE
> +flag.
> +The in-kernel module decompressor supports the following compression algorithms:
> +.P
> +.RS 4
> +.PD 0
> +.IP \[bu] 3
> +.I gzip
> +(since Linux 5.17)
> +.IP \[bu]
> +.I xz
> +(since Linux 5.17)
> +.IP \[bu]
> +.I zstd
> +.\" commit 169a58ad824d896b9e291a27193342616e651b82
> +(since Linux 6.2)
> +.PD
> +.RE
> +.P
> +The kernel only implements a single decompression method.
> +This is selected during module generation accordingly to the compression method
> +chosen in the kernel configuration.
> .SH RETURN VALUE
> On success, these system calls return 0.
> On error, \-1 is returned and
> @@ -223,10 +260,23 @@ is too large.
> .I flags
> is invalid.
> .TP
> +.B EINVAL
> +The decompressor sanity checks failed,
> +while loading a compressed module with flag
> +.B MODULE_INIT_COMPRESSED_FILE
> +set.
> +.TP
> .B ENOEXEC
> .I fd
> does not refer to an open file.
> .TP
> +.BR EOPNOTSUPP " (since Linux 5.17)"
> +The flag
> +.B MODULE_INIT_COMPRESSED_FILE
> +is set to load a compressed module,
> +and the kernel was built without
> +.BR CONFIG_MODULE_DECOMPRESS .
> +.TP
> .BR ETXTBSY " (since Linux 4.7)"
> .\" commit 39d637af5aa7577f655c58b9e55587566c63a0af
> The file referred to by
> --
> 2.39.2
>
>
--
<https://www.alejandro-colomar.es/>
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