1. 16 Jun, 2021 1 commit
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  3. 14 Jun, 2021 1 commit
    • Michael Ellerman's avatar
      powerpc/signal64: Copy siginfo before changing regs->nip · e41d6c3f
      Michael Ellerman authored
      In commit 96d7a4e0 ("powerpc/signal64: Rewrite handle_rt_signal64()
      to minimise uaccess switches") the 64-bit signal code was rearranged to
      use user_write_access_begin/end().
      
      As part of that change the call to copy_siginfo_to_user() was moved
      later in the function, so that it could be done after the
      user_write_access_end().
      
      In particular it was moved after we modify regs->nip to point to the
      signal trampoline. That means if copy_siginfo_to_user() fails we exit
      handle_rt_signal64() with an error but with regs->nip modified, whereas
      previously we would not modify regs->nip until the copy succeeded.
      
      Returning an error from signal delivery but with regs->nip updated
      leaves the process in a sort of half-delivered state. We do immediately
      force a SEGV in signal_setup_done(), called from do_signal(), so the
      process should never run in the half-delivered state.
      
      However that SEGV is not delivered until we've gone around to
      do_notify_resume() again, so it's possible some tracing could observe
      the half-delivered state.
      
      There are other cases where we fail signal delivery with regs partly
      updated, eg. the write to newsp and SA_SIGINFO, but the latter at least
      is very unlikely to fail as it reads back from the frame we just wrote
      to.
      
      Looking at other arches they seem to be more careful about leaving regs
      unchanged until the copy operations have succeeded, and in general that
      seems like good hygenie.
      
      So although the current behaviour is not cleary buggy, it's also not
      clearly correct. So move the call to copy_siginfo_to_user() up prior to
      the modification of regs->nip, which is closer to the old behaviour, and
      easier to reason about.
      
      Fixes: 96d7a4e0 ("powerpc/signal64: Rewrite handle_rt_signal64() to minimise uaccess switches")
      Signed-off-by: default avatarMichael Ellerman <mpe@ellerman.id.au>
      Link: https://lore.kernel.org/r/20210608134605.2783677-1-mpe@ellerman.id.au
      e41d6c3f
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