On Fri, Dec 8, 2017 at 6:06 AM, Peter Zijlstra <pet...@infradead.org> wrote: > On Fri, Dec 08, 2017 at 05:20:00AM -0800, Andy Lutomirski wrote: >> > >> > The error code of such an access is always 0x03. So I added a special >> > handler, which checks whether the address is in the LDT map range and >> > verifies that the access bit in the descriptor is 0. If that's the case it >> > sets it and returns. If not, the thing dies. That works. >> >> What if you are in kernel mode and try to return to a context with SS >> or CS pointing to a non-accessed segment? Or what if you try to >> schedule to a context with fs or, worse, gs pointing to such a >> segment? > > How would that be different from setting a 'crap' GS in modify_ldt() and > then returning from the syscall? That is something we should be able to > deal with already, no? > > Is this something ldt_gdt.c already tests? The current "Test GS" is in > test_gdt_invalidation() which seems to suggest not. > > Could we get a testcase for the exact situation you worry about? I'm not > sure I'd trust myself to get it right, all this LDT magic is new to me.
#GP on IRET is a failure, and we have disgusting code to handle it. #PF on IRET would not be a failure -- it's a case where IRET should be retried. Our crap that fixes up #GP would get that wrong and leave us with the wrong GSBASE.
