Messages in this thread | | | From | Blaisorblade <> | Subject | CONFIG_REGPARM - prevent_tail_call doubts (context: SKAS3 bug in detail) | Date | Tue, 12 Jul 2005 00:18:50 +0200 |
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I just diagnosed (and announced) a big bug affecting the SKAS3 patch: namely, syscall parameter values stored in registers may be corrupted for some syscalls on return, when called through int 0x80, and when CONFIG_REGPARM is enabled.
Ok, the diagnosys of the SKAS3 bug I just noticed is that simply, this construct:
int do_foo(params...) { }
asmlinkage int sys_foo(params...) { return do_foo(a_new_param, params...); }
does not work, because sys_foo() is optimized to reorder parameters on the stack and to tail-call do_foo. The corrupted parameters on the stack will then be restored (when calling with int $0x80) inside the userspace registers. From entry.S, especially from this comment:
/* if something modifies registers it must also disable sysexit */
it's clear that when using SYSENTER registers are not restored (even verified through sys_iopl() code, which touched EFLAGS).
I've used prevent_tail_call to fix this, and it works (verified with tests and assembly inspection). I even think I've understood why it works... it's clear why it disallows tail call, but I thought that GCC could create a normal call reusing some space from the stack frame of sys_foo, to create the stack frame of do_foo... it's just that it wouldn't improve speed.
This construct is used for four syscalls (sys_mmap2, old_mmap, sys_mprotect, sys_modify_ldt) and I verified the bug for all sys_mmap2 and sys_mprotect, and I'm sure about modify_ldt because the compiled code is identical to sys_mprotect().
I initially noticed this with the errno-vs-NPTL fix I and Al Viro discussed some time ago: it used indeed mmap2() and triggered the bug.
Luckily, strace reads the correct data (since syscall params are read before the syscall is done) so I couldn't do anything else than understand something bad was happening. -- Inform me of my mistakes, so I can keep imitating Homer Simpson's "Doh!". Paolo Giarrusso, aka Blaisorblade (Skype ID "PaoloGiarrusso", ICQ 215621894) http://www.user-mode-linux.org/~blaisorblade
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