Subject: Re: [PATCH] powerpc/64s: Fix possible corruption on big endian due to pgd/pud_present()

From: Balbir Singh <[email-masked]>
Date: Sat, 16 Feb 2019 21:55:11 +1100

On Thu, Feb 14, 2019 at 05:23:39PM +1100, Michael Ellerman wrote:
> In v4.20 we changed our pgd/pud_present() to check for _PAGE_PRESENT
> rather than just checking that the value is non-zero, e.g.:
> 
>   static inline int pgd_present(pgd_t pgd)
>   {
>  -       return !pgd_none(pgd);
>  +       return (pgd_raw(pgd) & cpu_to_be64(_PAGE_PRESENT));
>   }
> 
> Unfortunately this is broken on big endian, as the result of the
> bitwise && is truncated to int, which is always zero because

Not sure why that should happen, why is the result an int? What
causes the casting of pgd_t & be64 to be truncated to an int.

> _PAGE_PRESENT is 0x8000000000000000ul. This means pgd_present() and
> pud_present() are always false at compile time, and the compiler
> elides the subsequent code.
> 
> Remarkably with that bug present we are still able to boot and run
> with few noticeable effects. However under some work loads we are able
> to trigger a warning in the ext4 code:
> 
>   WARNING: CPU: 11 PID: 29593 at fs/ext4/inode.c:3927 .ext4_set_page_dirty+0x70/0xb0
>   CPU: 11 PID: 29593 Comm: debugedit Not tainted 4.20.0-rc1 #1
>   ...
>   NIP .ext4_set_page_dirty+0x70/0xb0
>   LR  .set_page_dirty+0xa0/0x150
>   Call Trace:
>    .set_page_dirty+0xa0/0x150
>    .unmap_page_range+0xbf0/0xe10
>    .unmap_vmas+0x84/0x130
>    .unmap_region+0xe8/0x190
>    .__do_munmap+0x2f0/0x510
>    .__vm_munmap+0x80/0x110
>    .__se_sys_munmap+0x14/0x30
>    system_call+0x5c/0x70
> 
> The fix is simple, we need to convert the result of the bitwise && to
> an int before returning it.
> 
> Thanks to Jan Kara and Aneesh for help with debugging.
> 
> Fixes: da7ad366b497 ("powerpc/mm/book3s: Update pmd_present to look at _PAGE_PRESENT bit")
> Cc: [email-masked] # v4.20+
> Reported-by: Erhard F. <[email-masked]>
> Reviewed-by: Aneesh Kumar K.V <[email-masked]>
> Signed-off-by: Michael Ellerman <[email-masked]>
> ---
>  arch/powerpc/include/asm/book3s/64/pgtable.h | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/powerpc/include/asm/book3s/64/pgtable.h b/arch/powerpc/include/asm/book3s/64/pgtable.h
> index c9bfe526ca9d..d8c8d7c9df15 100644
> --- a/arch/powerpc/include/asm/book3s/64/pgtable.h
> +++ b/arch/powerpc/include/asm/book3s/64/pgtable.h
> @@ -904,7 +904,7 @@ static inline int pud_none(pud_t pud)
>  
>  static inline int pud_present(pud_t pud)
>  {
> -	return (pud_raw(pud) & cpu_to_be64(_PAGE_PRESENT));
> +	return !!(pud_raw(pud) & cpu_to_be64(_PAGE_PRESENT));
>  }
>  
>  extern struct page *pud_page(pud_t pud);
> @@ -951,7 +951,7 @@ static inline int pgd_none(pgd_t pgd)
>  
>  static inline int pgd_present(pgd_t pgd)
>  {
> -	return (pgd_raw(pgd) & cpu_to_be64(_PAGE_PRESENT));
> +	return !!(pgd_raw(pgd) & cpu_to_be64(_PAGE_PRESENT));
>  }
>

Care to put a big FAT warning, so that we don't repeat this again
(as in authors planning on changing these bits). 

Balbir Singh.