• Viktor Rosendahl's avatar
    ARM: 6836/1: kprobes/fix emulation of LDR/STR instruction when Rn == PC · 0652f067
    Viktor Rosendahl authored
    The Rn value from the emulation is unconditionally written back;
    this is fine as long as Rn != PC because in that case, even if the
    instruction isn't a write back instruction, it will only result in the
    same value being written back.
    
    In case Rn == PC, then the emulated instruction doesn't have the
    actual PC value in Rn but an adjusted value; when this is written
    back, it will result in the PC being incorrectly updated.
    
    An altenative solution would be to check bits 24 and 22 to see whether
    the instruction actually is a write back instruction or not. I think
    it's enough to check whether Rn != PC,  because:
    - it's looks cheaper than the alternative
    - to my understaning it's not permitted to update the PC with a write
    back instruction, so we don't lose any ability to emulate legal
    instructions.
    - in case of writing back for non write back instructions where Rn != PC, it doesn't matter because the values are the same.
    
    Regarding the second point above, it would possibly be prudent to add
    some checking to prep_emulate_ldr_str(), so that instructions with
    both write back and Rn == PC would be rejected.
    Signed-off-by: default avatarViktor Rosendahl <viktor.rosendahl@nokia.com>
    Signed-off-by: default avatarRussell King <rmk+kernel@arm.linux.org.uk>
    0652f067
kprobes-decode.c 47.4 KB