• Roman Penyaev's avatar
    mm/vmalloc.c: make vmalloc_32_user() align base kernel virtual address to SHMLBA · 5a82ac71
    Roman Penyaev authored
    This patch repeats the original one from David S Miller:
    
      2dca6999 ("mm, perf_event: Make vmalloc_user() align base kernel virtual address to SHMLBA")
    
    but for missed vmalloc_32_user() case, which also requires correct
    alignment of virtual address on kernel side to avoid D-caches aliases.
    A bit of copy-paste from original patch to recover in memory of what is
    all about:
    
      When a vmalloc'd area is mmap'd into userspace, some kind of
      co-ordination is necessary for this to work on platforms with cpu
      D-caches which can have aliases.
    
      Otherwise kernel side writes won't be seen properly in userspace and
      vice versa.
    
      If the kernel side mapping and the user side one have the same
      alignment, modulo SHMLBA, this can work as long as VM_SHARED is shared
      of VMA and for all current users this is true. VM_SHARED will force
      SHMLBA alignment of the user side mmap on platforms with D-cache
      aliasing matters.
    
      David S. Miller
    
    > What are the user-visible runtime effects of this change?
    
    In simple words: proper alignment avoids possible difference in data,
    seen by different virtual mapings: userspace and kernel in our case.
    I.e. userspace reads cache line A, kernel writes to cache line B.  Both
    cache lines correspond to the same physical memory (thus aliases).
    
    So this should fix data corruption for archs with vivt and vipt caches,
    e.g. armv6.  Personally I've never worked with this archs, I just
    spotted the strange difference in code: for one case we do alignment,
    for another - not.  I have a strong feeling that David simply missed
    vmalloc_32_user() case.
    
    >
    > Is a -stable backport needed?
    
    No, I do not think so.  The only one user of vmalloc_32_user() is
    virtual frame buffer device drivers/video/fbdev/vfb.c, which has in the
    description "The main use of this frame buffer device is testing and
    debugging the frame buffer subsystem.  Do NOT enable it for normal
    systems!".
    
    And it seems to me that this vfb.c does not need 32bit addressable pages
    (vmalloc_32_user() case), because it is virtual device and should not
    care about things like dma32 zones, etc.  Probably is better to clean
    the code and switch vfb.c from vmalloc_32_user() to vmalloc_user() case
    and wipe out vmalloc_32_user() from vmalloc.c completely.  But I'm not
    very much sure that this is worth to do, that's so minor, so we can
    leave it as is.
    
    Link: http://lkml.kernel.org/r/20190108110944.23591-1-rpenyaev@suse.deSigned-off-by: default avatarRoman Penyaev <rpenyaev@suse.de>
    Reviewed-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Cc: Stephen Rothwell <sfr@canb.auug.org.au>
    Cc: Michal Hocko <mhocko@suse.com>
    Cc: David S. Miller <davem@davemloft.net>
    Cc: Peter Zijlstra <peterz@infradead.org>
    Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    5a82ac71
vmalloc.c 69.7 KB