Re: [PATCH] x86/kexec: fix double vfree of image->elf_headers

[Vlastimil Babka]

On 1/2/23 11:39, Vlastimil Babka wrote:
> An investigation of a "Trying to vfree() nonexistent vm area" bug
> occurring in arch_kimage_file_post_load_cleanup() doing a
> vfree(image->elf_headers) in our 5.14-based kernel yielded the following
> double vfree() scenario, also present in mainline:
> 
> SYSCALL_DEFINE5(kexec_file_load)
>   kimage_file_alloc_init()
>     kimage_file_prepare_segments()
>       arch_kexec_kernel_image_probe()
>         kexec_image_load_default()
>           kexec_bzImage64_ops.load()
>             bzImage64_load()
>               crash_load_segments()
>                 prepare_elf_headers(image, &kbuf.buffer, &kbuf.bufsz);
>                 image->elf_headers = kbuf.buffer;
> 		ret = kexec_add_buffer(&kbuf);
> 		if (ret) vfree((void *)image->elf_headers); // first vfree()
>       if (ret) kimage_file_post_load_cleanup()
>         vfree(image->elf_headers);                          // second vfree()
> 
> AFAICS the scenario is possible since v5.19 commit b3e34a47f989
> ("x86/kexec: fix memory leak of elf header buffer") that was marked for
> stable and also was backported to our kernel.
> 
> Fix the problem by setting the pointer to NULL after the first vfree().
> Also set elf_headers_sz to 0, as kimage_file_post_load_cleanup() does.
> 
> Fixes: b3e34a47f989 ("x86/kexec: fix memory leak of elf header buffer")
> Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
> Cc: Baoquan He <bhe@xxxxxxxxxx>
> Cc: Dave Young <dyoung@xxxxxxxxxx>
> Cc: <stable@xxxxxxxxxxxxxxx>

Takashi told me he sent a slightly different fix already in November:
https://lore.kernel.org/all/20221122115122.13937-1-tiwai@xxxxxxx/

Seems it wasn't picked up? You might pick his then, as Baoquan acked it, and
it's removing code, not adding it.

> ---
>  arch/x86/kernel/crash.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/arch/x86/kernel/crash.c b/arch/x86/kernel/crash.c
> index 9730c88530fc..0d651c05a49e 100644
> --- a/arch/x86/kernel/crash.c
> +++ b/arch/x86/kernel/crash.c
> @@ -403,6 +403,8 @@ int crash_load_segments(struct kimage *image)
>  	ret = kexec_add_buffer(&kbuf);
>  	if (ret) {
>  		vfree((void *)image->elf_headers);
> +		image->elf_headers = NULL;
> +		image->elf_headers_sz = 0;
>  		return ret;
>  	}
>  	image->elf_load_addr = kbuf.mem;