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

From: Michael Ellerman <[email-masked]>
Date: Sun, 17 Feb 2019 19:21:37 +1100 (AEDT)

On Thu, 2019-02-14 at 06:23:39 UTC, 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
> _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]>

Applied to powerpc fixes.

https://git.kernel.org/powerpc/c/a58007621be33e9f7c7bed5d5ff8ecb9

cheers