Commit fbbd7f1a authored Nov 20, 2017 by Heiko Carstens Committed by Martin Schwidefsky Dec 05, 2017

s390: always save and restore all registers on context switch



The switch_to() macro has an optimization to avoid saving and
restoring register contents that aren't needed for kernel threads.

There is however the possibility that a kernel thread execve's a user
space program. In such a case the execve'd process can partially see
the contents of the previous process, which shouldn't be allowed.

To avoid this, simply always save and restore register contents on
context switch.

Cc: <stable@vger.kernel.org> # v2.6.37+
Fixes: fdb6d070 ("switch_to: dont restore/save access & fpu regs for kernel threads")
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

parent bd7a9b37

arch/s390/include/asm/switch_to.h

+13 −14

Original line number	Diff line number	Diff line
		@@ -31,19 +31,18 @@ static inline void restore_access_regs(unsigned int *acrs)
		}

		#define switch_to(prev, next, last) do { \
		if (prev->mm) { \
		/* save_fpu_regs() sets the CIF_FPU flag, which enforces \
		* a restore of the floating point / vector registers as \
		* soon as the next task returns to user space \
		*/ \
		save_fpu_regs(); \
		save_access_regs(&prev->thread.acrs[0]); \
		save_ri_cb(prev->thread.ri_cb); \
		save_gs_cb(prev->thread.gs_cb); \
		} \
		update_cr_regs(next); \
		if (next->mm) { \
		set_cpu_flag(CIF_FPU); \
		restore_access_regs(&next->thread.acrs[0]); \
		restore_ri_cb(next->thread.ri_cb, prev->thread.ri_cb); \
		restore_gs_cb(next->thread.gs_cb); \
		} \
		prev = __switch_to(prev, next); \
		} while (0)