Messages in this thread | | | Date | Wed, 25 Jul 2018 17:37:26 +0100 | From | Will Deacon <> | Subject | Re: [PATCH 1/2] signal: Introduce COMPAT_SIGMINSTKSZ for use in compat_sys_sigaltstack |
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On Wed, Jul 25, 2018 at 04:54:27PM +0100, Dave Martin wrote: > On Wed, Jul 25, 2018 at 02:45:11PM +0100, Will Deacon wrote: > > @@ -3476,7 +3478,8 @@ int restore_altstack(const stack_t __user *uss) > > stack_t new; > > if (copy_from_user(&new, uss, sizeof(stack_t))) > > return -EFAULT; > > - (void)do_sigaltstack(&new, NULL, current_user_stack_pointer()); > > + (void)do_sigaltstack(&new, NULL, current_user_stack_pointer(), > > + MINSIGSTKSZ); > > Why can't this fail? > > If this fails here we silently go wrong, but... > > > /* squash all but EFAULT for now */ > > return 0; > > } > > @@ -3510,7 +3513,8 @@ static int do_compat_sigaltstack(const compat_stack_t __user *uss_ptr, > > uss.ss_size = uss32.ss_size; > > } > > ret = do_sigaltstack(uss_ptr ? &uss : NULL, &uoss, > > - compat_user_stack_pointer()); > > + compat_user_stack_pointer(), > > + COMPAT_MINSIGSTKSZ); > > If this fails on arm64, we seem to SEGV (see compat_sys_rt_sigreturn()). > > This patch doesn't introduce this inconsistency, this might be a good > opportunity to clean it up.
I don't think there's an inconsistency here -- both restore_altstack and compat_restore_altstack suppress all non--EFAULT errors (remember that uoss is NULL in both cases, so the copy_from_user() immediately before the do_sigaltstack() call for the native path is all we care about). I think the behaviour is: on a sigreturn, if you set the altstack to be an unmapped address then you get a SEGV, otherwise if you make it invalid in some other way (e.g. too small) then it's ignored and the old altstack remains intact.
Will
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