[PATCH] 5/6 i386 non-reversible-fs-gs
Date: Fri Jul 29 2005 - 23:18:44 EST
Subtle fix: load_TLS has been moved after saving %fs and %gs segments to
avoid creating non-reversible segments. This could conceivably cause a bug
if the kernel ever needed to save and restore fs/gs from the NMI handler.
It currently does not, but this is the safest approach to avoiding fs/gs
corruption. SMIs are safe, since SMI saves the descriptor hidden state.
Diffs against: patch-2.6.13-rc4 + cpu-inline-cleanup + dt-inline-cleanup
Signed-off-by: Zachary Amsden <zach@xxxxxxxxxx>
--- linux-2.6.13.orig/arch/i386/kernel/process.c 2005-07-29 11:17:02.000000000 -0700
+++ linux-2.6.13/arch/i386/kernel/process.c 2005-07-29 11:50:19.000000000 -0700
@@ -678,21 +678,26 @@
- * Reload esp0, LDT and the page table pointer:
+ * Reload esp0.
- * Load the per-thread Thread-Local Storage descriptor.
+ * Save away %fs and %gs. No need to save %es and %ds, as
+ * those are always kernel segments while inside the kernel.
+ * Doing this before setting the new TLS descriptors avoids
+ * the situation where we temporarily have non-reloadable
+ * segments in %fs and %gs. This could be an issue if the
+ * NMI handler ever used %fs or %gs (it does not today), or
+ * if the kernel is running inside of a hypervisor layer.
- load_TLS(next, cpu);
+ savesegment(fs, prev->fs);
+ savesegment(gs, prev->gs);
- * Save away %fs and %gs. No need to save %es and %ds, as
- * those are always kernel segments while inside the kernel.
+ * Load the per-thread Thread-Local Storage descriptor.
- asm volatile("mov %%fs,%0":"=m" (prev->fs));
- asm volatile("mov %%gs,%0":"=m" (prev->gs));
+ load_TLS(next, cpu);
* Restore %fs and %gs if needed.
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