Re: [PATCH] x86/speculation/l1tf: fix overflow on l1tf_pfn_limit() on 32bit

[Michal Hocko]

On Mon 20-08-18 11:58:35, Vlastimil Babka wrote:
> On 32bit PAE kernels on 64bit hardware with enough physical bits,
> l1tf_pfn_limit() will overflow unsigned long. This in turn affects
> max_swapfile_size() and can lead to swapon returning -EINVAL. This has been
> observed in a 32bit guest with 42 bits physical address size, where
> max_swapfile_size() overflows exactly to 1 << 32, thus zero, and produces the
> following warning to dmesg:
> 
> [    6.396845] Truncating oversized swap area, only using 0k out of 2047996k
> 
> Fix this by using unsigned long long instead.
> 
> Reported-by: Dominique Leuenberger <dimstar@xxxxxxx>
> Reported-by: Adrian Schroeter <adrian@xxxxxxx>
> Fixes: 17dbca119312 ("x86/speculation/l1tf: Add sysfs reporting for l1tf")
> Fixes: 377eeaa8e11f ("x86/speculation/l1tf: Limit swap file size to MAX_PA/2")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>

Looks good to me. I would probably use phys_addr_t which would be more
descriptive but this is just minor thing.

Acked-by: Michal Hocko <mhocko@xxxxxxxx>

> ---
>  arch/x86/include/asm/processor.h | 4 ++--
>  arch/x86/mm/init.c               | 4 ++--
>  2 files changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
> index 682286aca881..a0a52274cb4a 100644
> --- a/arch/x86/include/asm/processor.h
> +++ b/arch/x86/include/asm/processor.h
> @@ -181,9 +181,9 @@ extern const struct seq_operations cpuinfo_op;
>  
>  extern void cpu_detect(struct cpuinfo_x86 *c);
>  
> -static inline unsigned long l1tf_pfn_limit(void)
> +static inline unsigned long long l1tf_pfn_limit(void)
>  {
> -	return BIT(boot_cpu_data.x86_phys_bits - 1 - PAGE_SHIFT) - 1;
> +	return BIT_ULL(boot_cpu_data.x86_phys_bits - 1 - PAGE_SHIFT) - 1;
>  }
>  
>  extern void early_cpu_init(void);
> diff --git a/arch/x86/mm/init.c b/arch/x86/mm/init.c
> index acfab322fbe0..02de3d6065c4 100644
> --- a/arch/x86/mm/init.c
> +++ b/arch/x86/mm/init.c
> @@ -923,7 +923,7 @@ unsigned long max_swapfile_size(void)
>  
>  	if (boot_cpu_has_bug(X86_BUG_L1TF)) {
>  		/* Limit the swap file size to MAX_PA/2 for L1TF workaround */
> -		unsigned long l1tf_limit = l1tf_pfn_limit() + 1;
> +		unsigned long long l1tf_limit = l1tf_pfn_limit() + 1;
>  		/*
>  		 * We encode swap offsets also with 3 bits below those for pfn
>  		 * which makes the usable limit higher.
> @@ -931,7 +931,7 @@ unsigned long max_swapfile_size(void)
>  #if CONFIG_PGTABLE_LEVELS > 2
>  		l1tf_limit <<= PAGE_SHIFT - SWP_OFFSET_FIRST_BIT;
>  #endif
> -		pages = min_t(unsigned long, l1tf_limit, pages);
> +		pages = min_t(unsigned long long, l1tf_limit, pages);
>  	}
>  	return pages;
>  }
> -- 
> 2.18.0

-- 
Michal Hocko
SUSE Labs