[PATCH] i386: load_tls() fix

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.

Signed-off-by: Zachary Amsden <zach@vmware.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 12 insertions, 7 deletions

diff --git a/arch/i386/kernel/process.c b/arch/i386/kernel/process.c
index 761d4ed47ef3..9d94995e9672 100644
--- a/arch/i386/kernel/process.c
+++ b/arch/i386/kernel/process.c
@@ -678,21 +678,26 @@ struct task_struct fastcall * __switch_to(struct task_struct *prev_p, struct tas
         __unlazy_fpu(prev_p);
 
         /*
-         * Reload esp0, LDT and the page table pointer:
+         * Reload esp0.
          */
         load_esp0(tss, next);
 
         /*
-         * 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.