linux-kernel - Re: [PATCH v2 1/1] x86/boot/compressed: Fix avoiding memmap in physical KASLR

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Message-ID: <CAMj1kXFqwOYBNPzNYo2vjPycjyO5SrEnk_wRVDw_dhGd8qT2qQ@mail.gmail.com>
Date: Thu, 23 Oct 2025 10:24:54 +0200
From: Ard Biesheuvel <ardb@...nel.org>
To: Michal Clapinski <mclapinski@...gle.com>
Cc: Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, Chris Li <chrisl@...nel.org>, x86@...nel.org, 
	"H. Peter Anvin" <hpa@...or.com>, Dan Williams <dan.j.williams@...el.com>, 
	Pasha Tatashin <pasha.tatashin@...een.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/1] x86/boot/compressed: Fix avoiding memmap in
 physical KASLR

Hi Michal,

Thanks for the patch.

On Thu, 23 Oct 2025 at 01:37, Michal Clapinski <mclapinski@...gle.com> wrote:
>
> The intent of the code was to cancel KASLR if there are more than 4
> memmap args. Unfortunately, it was only doing that if the memmap args
> were comma delimited, not if they were entirely separate.
> So it would disable physical KASLR for:
> memmap=1G!4G,1G!5G,1G!6G,1G!7G,1G!8G
> since the whole function is just called once and we hit the `if` at
> the end of the function.
>
> But it would not disable physical KASLR for:
> memmap=1G!4G memmap=1G!5G memmap=1G!6G memmap=1G!7G memmap=1G!8G
> since the whole function would be called 5 times and the last `if`
> would never trigger.
>
> For the second input, the code would avoid the first 4 memmap regions
> but not the last one (it could put the kernel there).
>
> The new code disables physical KASLR for both of those inputs.
>

Should we just disable physical KASLR if memmap= appears at all?

> Signed-off-by: Michal Clapinski <mclapinski@...gle.com>
> Suggested-by: Chris Li <chrisl@...nel.org>
> Fixes: d52e7d5a952c ("x86/KASLR: Parse all 'memmap=' boot option entries")
> ---
> The patch was suggested by Chris and I modified it a little without his
> knowledge. I don't know which tags are appropriate.

I think this is fine, unless Chris has a different opinion? In any
case, you might add a link to the original submission.


> ---
>  arch/x86/boot/compressed/kaslr.c | 18 +++++++++---------
>  1 file changed, 9 insertions(+), 9 deletions(-)
>
> diff --git a/arch/x86/boot/compressed/kaslr.c b/arch/x86/boot/compressed/kaslr.c
> index 3b0948ad449f..649264503ce6 100644
> --- a/arch/x86/boot/compressed/kaslr.c
> +++ b/arch/x86/boot/compressed/kaslr.c
> @@ -162,14 +162,18 @@ static void mem_avoid_memmap(char *str)
>  {
>         static int i;
>
> -       if (i >= MAX_MEMMAP_REGIONS)
> -               return;
> -
> -       while (str && (i < MAX_MEMMAP_REGIONS)) {
> +       while (str) {
>                 int rc;
>                 u64 start, size;
> -               char *k = strchr(str, ',');
> +               char *k;
> +
> +               if (i >= MAX_MEMMAP_REGIONS) {
> +                       /* Too many memmap regions, disable physical KASLR. */
> +                       memmap_too_large = true;
> +                       return;
> +               }
>
> +               k = strchr(str, ',');
>                 if (k)
>                         *k++ = 0;
>
> @@ -190,10 +194,6 @@ static void mem_avoid_memmap(char *str)
>                 mem_avoid[MEM_AVOID_MEMMAP_BEGIN + i].size = size;
>                 i++;
>         }
> -
> -       /* More than 4 memmaps, fail kaslr */
> -       if ((i >= MAX_MEMMAP_REGIONS) && str)
> -               memmap_too_large = true;
>  }
>
>  /* Store the number of 1GB huge pages which users specified: */
> --
> 2.51.1.814.gb8fa24458f-goog
>