Am 18.06.2024 um 21:58 schrieb Luis Chamberlain:
On Thu, Jun 06, 2024 at 03:31:49PM +0200, Daniel v. Kirschten wrote:
If a module is being loaded, and the .gnu.linkonce.this_module section
in the module's ELF file does not have the WRITE flag, the kernel will
map the finished module struct of that module as read-only.
This causes a kernel panic when the struct is written to the first time
after it has been marked read-only. Currently this happens in
complete_formation in kernel/module/main.c:2765 when the module's state is
set to MODULE_STATE_COMING, just after setting up the memory protections.
How did you find this issue?
In a university course I got the assignment to manually craft a loadable
.ko file, given only a regular object file, without using Kbuild. During
testing my module files, most of them were simply (correctly) rejected
by the kernel with an appropriate error message, but at some point I ran
into this exact kernel panic, and investigated it to understand why my
module file was invalid.
Down the line, this seems to lead to unpredictable freezes when trying to
load other modules - I guess this is due to some structures not being
cleaned up properly, but I didn't investigate this further.
A check already exists which verifies that .gnu.linkonce.this_module
is ALLOC. This patch simply adds an analogous check for WRITE.
Can you check to ensure our modules generated have a respective check to
ensure this check exists at build time? That would proactively inform
userspace when a built module is not built correctly, and the tool
responsible can be identified.
See above - I don't think it's possible to create such a broken module
file with any of "official" tools. I haven't looked too deeply into how
Kbuild actually builds modules, but as far as I know, the user doesn't
even come into contact with this_module when using the regular
toolchain, because Kbuild is responsible for creating the .this_module
section. And Kbuild of course creates it with the correct flags. So if I
understand correctly, this problem can only occur when the module was
built by some external tooling (or manually, in my case).
Daniel
