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dc4c2a0aed
The espfix code triggers if we have a protected mode userspace application with a 16-bit stack. On returning to userspace, with iret, the CPU doesn't restore the high word of the stack pointer. This is an "official" bug, and the work-around used in the kernel is to temporarily switch to a 32-bit stack segment/pointer pair where the high word of the pointer is equal to the high word of the userspace stackpointer. The current implementation uses THREAD_SIZE to determine the cut-off, but there is no good reason not to use the more natural 64kb... However, implementing this by simply substituting THREAD_SIZE with 65536 in patch_espfix_desc crashed the test application. patch_espfix_desc tries to do what is described above, but gets it subtly wrong if the userspace stack pointer is just below a multiple of THREAD_SIZE: an overflow occurs to bit 13... With a bit of luck, when the kernelspace stackpointer is just below a 64kb-boundary, the overflow then ripples trough to bit 16 and userspace will see its stack pointer changed by 65536. This patch moves all espfix code into entry_32.S. Selecting a 16-bit cut-off simplifies the code. The game with changing the limit dynamically is removed too. It complicates matters and I see no value in it. Changing only the top 16-bit word of ESP is one instruction and it also implies that only two bytes of the ESPFIX GDT entry need to be changed and this can be implemented in just a handful simple to understand instructions. As a side effect, the operation to compute the original ESP from the ESPFIX ESP and the GDT entry simplifies a bit too, and the remaining three instructions have been expanded inline in entry_32.S. impact: can now reliably run userspace with ESP=xxxxfffc on 16-bit stack segment Signed-off-by: Alexander van Heukelum <heukelum@fastmail.fm> Acked-by: Stas Sergeev <stsp@aknet.ru> Signed-off-by: H. Peter Anvin <hpa@zytor.com> |
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