On Tue, Dec 22, 2020 at 07:31:24PM +0800, Pan Zhang wrote:
> When the kernel is loading,
> the load address of the kernel needs to be calculated firstly.
>
> If the kernel address space layout randomization(`kaslr`) is enabled,
> the memory range reserved by the memmap parameter will be excluded
> to avoid loading the kernel address into the memmap reserved area.
>
> Currently, this is what the manual says:
> `memmap = nn [KMG] @ss [KMG]
> [KNL] Force usage of a specific region of memory.
> Region of memory to be used is from ss to ss + nn.
> If @ss [KMG] is omitted, it is equivalent to mem = nn [KMG],
> which limits max address to nn [KMG].
> Multiple different regions can be specified,
> comma delimited.
> Example:
> memmap=100M@2G, 100M#3G, 1G!1024G
> `
>
> Can we relax the use of memmap?
> In our production environment we see many people who use it like this:
> Separate multiple memmaps parameters to reserve memory,
> memmap=xx\$xxx memmap=xx\$xxx memmap=xx\$xxx memmap=xx\$xxx memmap=xx\$xxx
>
> If this format is used, and the reserved memory segment is greater than 4,
> there is no way to parse the 5th and subsequent memmaps and the kaslr cannot
> be disabled by `memmap_too_large`
> so the kernel loading address may fall within the memmap range
> (reserved memory area from memmap after fourth segment),
> which will have bad consequences for use of reserved memory.
>
> Signed-off-by: Pan Zhang <zhangpa...@huawei.com>
> ---
> arch/x86/boot/compressed/kaslr.c | 5 +----
> 1 file changed, 1 insertion(+), 4 deletions(-)
>
> diff --git a/arch/x86/boot/compressed/kaslr.c
> b/arch/x86/boot/compressed/kaslr.c
> index d7408af..24a2778 100644
> --- a/arch/x86/boot/compressed/kaslr.c
> +++ b/arch/x86/boot/compressed/kaslr.c
> @@ -203,9 +203,6 @@ static void mem_avoid_memmap(enum parse_mode mode, char
> *str)
> {
> static int i;
>
> - if (i >= MAX_MEMMAP_REGIONS)
> - return;
> -
> while (str && (i < MAX_MEMMAP_REGIONS)) {
> int rc;
> unsigned long long start, size;
> @@ -233,7 +230,7 @@ static void mem_avoid_memmap(enum parse_mode mode, char
> *str)
> }
>
> /* More than 4 memmaps, fail kaslr */
> - if ((i >= MAX_MEMMAP_REGIONS) && str)
> + if ((i >= MAX_MEMMAP_REGIONS) && !memmap_too_large)
I think this should stay the way it was, otherwise KASLR will be
disabled even if exactly MAX_MEMMAP_REGIONS were specified. Removing the
early return as you did above should be enough to cause the flag to be
set if a 5th memmap is specified in a separate parameter, right?
> memmap_too_large = true;
> }
>
> --
> 2.7.4
>
