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x86/asm, sched/x86: Rewrite the FS and GS context switch code

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The old code was incomprehensible and was buggy on AMD CPUs.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Reviewed-by: Borislav Petkov <bp@suse.de>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Rudolf Marek <r.marek@assembler.cz>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/5f6bde874c6fe6831c6711b5b1522a238ba035b4.1460075211.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Andy Lutomirski 2016-04-07 17:31:47 -07:00 committed by Ingo Molnar
parent 7a5d670487
commit 3e2b68d752
1 changed files with 93 additions and 55 deletions
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148
arch/x86/kernel/process_64.c
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@ -282,7 +282,7 @@ __switch_to(struct task_struct *prev_p, struct task_struct *next_p)
struct fpu *next_fpu = &next->fpu;
int cpu = smp_processor_id();
struct tss_struct *tss = &per_cpu(cpu_tss, cpu);
unsigned fsindex, gsindex;
unsigned prev_fsindex, prev_gsindex;
fpu_switch_t fpu_switch;
fpu_switch = switch_fpu_prepare(prev_fpu, next_fpu, cpu);
@ -292,8 +292,8 @@ __switch_to(struct task_struct *prev_p, struct task_struct *next_p)
*
* (e.g. xen_load_tls())
*/
savesegment(fs, fsindex);
savesegment(gs, gsindex);
savesegment(fs, prev_fsindex);
savesegment(gs, prev_gsindex);
/*
* Load TLS before restoring any segments so that segment loads
@ -336,66 +336,104 @@ __switch_to(struct task_struct *prev_p, struct task_struct *next_p)
* Switch FS and GS.
*
* These are even more complicated than DS and ES: 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.
* 64-bit bases are that controlled by arch_prctl. The bases
* don't necessarily match the selectors, as user code can do
* any number of things to cause them to be inconsistent.
*
* 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.
* We don't promise to preserve the bases if the selectors are
* nonzero. We also don't promise to preserve the base if the
* selector is zero and the base doesn't match whatever was
* most recently passed to ARCH_SET_FS/GS. (If/when the
* FSGSBASE instructions are enabled, we'll need to offer
* stronger guarantees.)
*
* 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.
* As an invariant,
* (fs != 0 && fsindex != 0) || (gs != 0 && gsindex != 0) is
* impossible.
*/
if (unlikely(fsindex | next->fsindex | prev->fs)) {
if (next->fsindex) {
/* Loading a nonzero value into FS sets the index and base. */
loadsegment(fs, next->fsindex);
/*
* 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;
} else {
if (next->fs) {
/* Next index is zero but next base is nonzero. */
if (prev_fsindex)
loadsegment(fs, 0);
wrmsrl(MSR_FS_BASE, next->fs);
} else {
/* Next base and index are both zero. */
if (static_cpu_has_bug(X86_BUG_NULL_SEG)) {
/*
* We don't know the previous base and can't
* find out without RDMSR. Forcibly clear it.
*/
loadsegment(fs, __USER_DS);
loadsegment(fs, 0);
} else {
/*
* If the previous index is zero and ARCH_SET_FS
* didn't change the base, then the base is
* also zero and we don't need to do anything.
*/
if (prev->fs || prev_fsindex)
loadsegment(fs, 0);
}
}
}
if (next->fs)
wrmsrl(MSR_FS_BASE, next->fs);
prev->fsindex = fsindex;
/*
* Save the old state and preserve the invariant.
* NB: if prev_fsindex == 0, then we can't reliably learn the base
* without RDMSR because Intel user code can zero it without telling
* us and AMD user code can program any 32-bit value without telling
* us.
*/
if (prev_fsindex)
prev->fs = 0;
prev->fsindex = prev_fsindex;
if (unlikely(gsindex | next->gsindex | prev->gs)) {
if (next->gsindex) {
/* Loading a nonzero value into GS sets the index and base. */
load_gs_index(next->gsindex);
/* This works (and fails) the same way as fsindex above. */
if (gsindex)
prev->gs = 0;
} else {
if (next->gs) {
/* Next index is zero but next base is nonzero. */
if (prev_gsindex)
load_gs_index(0);
wrmsrl(MSR_KERNEL_GS_BASE, next->gs);
} else {
/* Next base and index are both zero. */
if (static_cpu_has_bug(X86_BUG_NULL_SEG)) {
/*
* We don't know the previous base and can't
* find out without RDMSR. Forcibly clear it.
*
* This contains a pointless SWAPGS pair.
* Fixing it would involve an explicit check
* for Xen or a new pvop.
*/
load_gs_index(__USER_DS);
load_gs_index(0);
} else {
/*
* If the previous index is zero and ARCH_SET_GS
* didn't change the base, then the base is
* also zero and we don't need to do anything.
*/
if (prev->gs || prev_gsindex)
load_gs_index(0);
}
}
}
if (next->gs)
wrmsrl(MSR_KERNEL_GS_BASE, next->gs);
prev->gsindex = gsindex;
/*
* Save the old state and preserve the invariant.
* NB: if prev_gsindex == 0, then we can't reliably learn the base
* without RDMSR because Intel user code can zero it without telling
* us and AMD user code can program any 32-bit value without telling
* us.
*/
if (prev_gsindex)
prev->gs = 0;
prev->gsindex = prev_gsindex;
switch_fpu_finish(next_fpu, fpu_switch);