• David Miller's avatar
    [FUTEX] Fix address computation in compat code. · 3c5fd9c7
    David Miller authored
    compat_exit_robust_list() computes a pointer to the
    futex entry in userspace as follows:
    
    	(void __user *)entry + futex_offset
    
    'entry' is a 'struct robust_list __user *', and
    'futex_offset' is a 'compat_long_t' (typically a 's32').
    
    Things explode if the 32-bit sign bit is set in futex_offset.
    
    Type promotion sign extends futex_offset to a 64-bit value before
    adding it to 'entry'.
    
    This triggered a problem on sparc64 running 32-bit applications which
    would lock up a cpu looping forever in the fault handling for the
    userspace load in handle_futex_death().
    
    Compat userspace runs with address masking (wherein the cpu zeros out
    the top 32-bits of every effective address given to a memory operation
    instruction) so the sparc64 fault handler accounts for this by
    zero'ing out the top 32-bits of the fault address too.
    
    Since the kernel properly uses the compat_uptr interfaces, kernel side
    accesses to compat userspace work too since they will only use
    addresses with the top 32-bit clear.
    
    Because of this compat futex layer bug we get into the following loop
    when executing the get_user() load near the top of handle_futex_death():
    
    1) load from address '0xfffffffff7f16bd8', FAULT
    2) fault handler clears upper 32-bits, processes fault
       for address '0xf7f16bd8' which succeeds
    3) goto #1
    
    I want to thank Bernd Zeimetz, Josip Rodin, and Fabio Massimo Di Nitto
    for their tireless efforts helping me track down this bug.
    Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    3c5fd9c7
futex_compat.c 4.18 KB