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-rw-r--r--arch/x86/kernel/process_64.c101
1 files changed, 73 insertions, 28 deletions
diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
index 3ed4a68d4013..5a2c02913af3 100644
--- a/arch/x86/kernel/process_64.c
+++ b/arch/x86/kernel/process_64.c
@@ -283,24 +283,9 @@ __switch_to(struct task_struct *prev_p, struct task_struct *next_p)
fpu = switch_fpu_prepare(prev_p, next_p, cpu);
- /*
- * Reload esp0, LDT and the page table pointer:
- */
+ /* Reload esp0 and ss1. */
load_sp0(tss, next);
- /*
- * Switch DS and ES.
- * This won't pick up thread selector changes, but I guess that is ok.
- */
- savesegment(es, prev->es);
- if (unlikely(next->es | prev->es))
- loadsegment(es, next->es);
-
- savesegment(ds, prev->ds);
- if (unlikely(next->ds | prev->ds))
- loadsegment(ds, next->ds);
-
-
/* We must save %fs and %gs before load_TLS() because
* %fs and %gs may be cleared by load_TLS().
*
@@ -309,41 +294,101 @@ __switch_to(struct task_struct *prev_p, struct task_struct *next_p)
savesegment(fs, fsindex);
savesegment(gs, gsindex);
+ /*
+ * Load TLS before restoring any segments so that segment loads
+ * reference the correct GDT entries.
+ */
load_TLS(next, cpu);
/*
- * Leave lazy mode, flushing any hypercalls made here.
- * This must be done before restoring TLS segments so
- * the GDT and LDT are properly updated, and must be
- * done before math_state_restore, so the TS bit is up
- * to date.
+ * Leave lazy mode, flushing any hypercalls made here. This
+ * must be done after loading TLS entries in the GDT but before
+ * loading segments that might reference them, and and it must
+ * be done before math_state_restore, so the TS bit is up to
+ * date.
*/
arch_end_context_switch(next_p);
+ /* Switch DS and ES.
+ *
+ * Reading them only returns the selectors, but writing them (if
+ * nonzero) loads the full descriptor from the GDT or LDT. The
+ * LDT for next is loaded in switch_mm, and the GDT is loaded
+ * above.
+ *
+ * We therefore need to write new values to the segment
+ * registers on every context switch unless both the new and old
+ * values are zero.
+ *
+ * Note that we don't need to do anything for CS and SS, as
+ * those are saved and restored as part of pt_regs.
+ */
+ savesegment(es, prev->es);
+ if (unlikely(next->es | prev->es))
+ loadsegment(es, next->es);
+
+ savesegment(ds, prev->ds);
+ if (unlikely(next->ds | prev->ds))
+ loadsegment(ds, next->ds);
+
/*
* Switch FS and GS.
*
- * Segment register != 0 always requires a reload. Also
- * reload when it has changed. When prev process used 64bit
- * base always reload to avoid an information leak.
+ * These are even more complicated than FS and GS: they have
+ * 64-bit bases are that controlled by arch_prctl. Those bases
+ * only differ from the values in the GDT or LDT if the selector
+ * is 0.
+ *
+ * Loading the segment register resets the hidden base part of
+ * the register to 0 or the value from the GDT / LDT. If the
+ * next base address zero, writing 0 to the segment register is
+ * much faster than using wrmsr to explicitly zero the base.
+ *
+ * The thread_struct.fs and thread_struct.gs values are 0
+ * if the fs and gs bases respectively are not overridden
+ * from the values implied by fsindex and gsindex. They
+ * are nonzero, and store the nonzero base addresses, if
+ * the bases are overridden.
+ *
+ * (fs != 0 && fsindex != 0) || (gs != 0 && gsindex != 0) should
+ * be impossible.
+ *
+ * Therefore we need to reload the segment registers if either
+ * the old or new selector is nonzero, and we need to override
+ * the base address if next thread expects it to be overridden.
+ *
+ * This code is unnecessarily slow in the case where the old and
+ * new indexes are zero and the new base is nonzero -- it will
+ * unnecessarily write 0 to the selector before writing the new
+ * base address.
+ *
+ * Note: This all depends on arch_prctl being the only way that
+ * user code can override the segment base. Once wrfsbase and
+ * wrgsbase are enabled, most of this code will need to change.
*/
if (unlikely(fsindex | next->fsindex | prev->fs)) {
loadsegment(fs, next->fsindex);
+
/*
- * Check if the user used a selector != 0; if yes
- * clear 64bit base, since overloaded base is always
- * mapped to the Null selector
+ * If user code wrote a nonzero value to FS, then it also
+ * cleared the overridden base address.
+ *
+ * XXX: if user code wrote 0 to FS and cleared the base
+ * address itself, we won't notice and we'll incorrectly
+ * restore the prior base address next time we reschdule
+ * the process.
*/
if (fsindex)
prev->fs = 0;
}
- /* when next process has a 64bit base use it */
if (next->fs)
wrmsrl(MSR_FS_BASE, next->fs);
prev->fsindex = fsindex;
if (unlikely(gsindex | next->gsindex | prev->gs)) {
load_gs_index(next->gsindex);
+
+ /* This works (and fails) the same way as fsindex above. */
if (gsindex)
prev->gs = 0;
}