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Subject[tip: x86/iopl] x86/ioperm: Move TSS bitmap update to exit to user work
The following commit has been merged into the x86/iopl branch of tip:

Commit-ID: 8ef04e97f1b49edd583e7b6fe81a3e24a3916786
Gitweb: https://git.kernel.org/tip/8ef04e97f1b49edd583e7b6fe81a3e24a3916786
Author: Thomas Gleixner <tglx@linutronix.de>
AuthorDate: Wed, 13 Nov 2019 21:42:52 +01:00
Committer: Thomas Gleixner <tglx@linutronix.de>
CommitterDate: Thu, 14 Nov 2019 20:15:04 +01:00

x86/ioperm: Move TSS bitmap update to exit to user work

There is no point to update the TSS bitmap for tasks which use I/O bitmaps
on every context switch. It's enough to update it right before exiting to
user space.

That reduces the context switch bitmap handling to invalidating the io
bitmap base offset in the TSS when the outgoing task has TIF_IO_BITMAP
set. The invaldiation is done on purpose when a task with an IO bitmap
switches out to prevent any possible leakage of an activated IO bitmap.

It also removes the requirement to update the tasks bitmap atomically in
ioperm().

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20191113210104.882617091@linutronix.de


---
arch/x86/entry/common.c | 4 ++-
arch/x86/include/asm/io_bitmap.h | 2 +-
arch/x86/include/asm/thread_info.h | 9 ++--
arch/x86/kernel/ioport.c | 25 +-----------
arch/x86/kernel/process.c | 59 +++++++++++++++++++----------
5 files changed, 54 insertions(+), 45 deletions(-)

diff --git a/arch/x86/entry/common.c b/arch/x86/entry/common.c
index 3f8e226..9747876 100644
--- a/arch/x86/entry/common.c
+++ b/arch/x86/entry/common.c
@@ -33,6 +33,7 @@
#include <asm/cpufeature.h>
#include <asm/fpu/api.h>
#include <asm/nospec-branch.h>
+#include <asm/io_bitmap.h>

#define CREATE_TRACE_POINTS
#include <trace/events/syscalls.h>
@@ -196,6 +197,9 @@ __visible inline void prepare_exit_to_usermode(struct pt_regs *regs)
/* Reload ti->flags; we may have rescheduled above. */
cached_flags = READ_ONCE(ti->flags);

+ if (unlikely(cached_flags & _TIF_IO_BITMAP))
+ tss_update_io_bitmap();
+
fpregs_assert_state_consistent();
if (unlikely(cached_flags & _TIF_NEED_FPU_LOAD))
switch_fpu_return();
diff --git a/arch/x86/include/asm/io_bitmap.h b/arch/x86/include/asm/io_bitmap.h
index d63bd5a..6d82a37 100644
--- a/arch/x86/include/asm/io_bitmap.h
+++ b/arch/x86/include/asm/io_bitmap.h
@@ -11,4 +11,6 @@ struct io_bitmap {
unsigned long bitmap[IO_BITMAP_LONGS];
};

+void tss_update_io_bitmap(void);
+
#endif
diff --git a/arch/x86/include/asm/thread_info.h b/arch/x86/include/asm/thread_info.h
index f945353..0accf44 100644
--- a/arch/x86/include/asm/thread_info.h
+++ b/arch/x86/include/asm/thread_info.h
@@ -143,8 +143,8 @@ struct thread_info {
_TIF_NOHZ)

/* flags to check in __switch_to() */
-#define _TIF_WORK_CTXSW_BASE \
- (_TIF_IO_BITMAP|_TIF_NOCPUID|_TIF_NOTSC|_TIF_BLOCKSTEP| \
+#define _TIF_WORK_CTXSW_BASE \
+ (_TIF_NOCPUID | _TIF_NOTSC | _TIF_BLOCKSTEP | \
_TIF_SSBD | _TIF_SPEC_FORCE_UPDATE)

/*
@@ -156,8 +156,9 @@ struct thread_info {
# define _TIF_WORK_CTXSW (_TIF_WORK_CTXSW_BASE)
#endif

-#define _TIF_WORK_CTXSW_PREV (_TIF_WORK_CTXSW|_TIF_USER_RETURN_NOTIFY)
-#define _TIF_WORK_CTXSW_NEXT (_TIF_WORK_CTXSW)
+#define _TIF_WORK_CTXSW_PREV (_TIF_WORK_CTXSW| _TIF_USER_RETURN_NOTIFY | \
+ _TIF_IO_BITMAP)
+#define _TIF_WORK_CTXSW_NEXT (_TIF_WORK_CTXSW)

#define STACK_WARN (THREAD_SIZE/8)

diff --git a/arch/x86/kernel/ioport.c b/arch/x86/kernel/ioport.c
index 7c1ab5c..198bead 100644
--- a/arch/x86/kernel/ioport.c
+++ b/arch/x86/kernel/ioport.c
@@ -21,9 +21,8 @@ static atomic64_t io_bitmap_sequence;
*/
long ksys_ioperm(unsigned long from, unsigned long num, int turn_on)
{
- unsigned int i, max_long, bytes, bytes_updated;
struct thread_struct *t = &current->thread;
- struct tss_struct *tss;
+ unsigned int i, max_long;
struct io_bitmap *iobm;

if ((from + num <= from) || (from + num > IO_BITMAP_BITS))
@@ -50,10 +49,9 @@ long ksys_ioperm(unsigned long from, unsigned long num, int turn_on)
}

/*
- * Update the bitmap and the TSS copy with preemption disabled to
- * prevent a race against context switch.
+ * Update the tasks bitmap. The update of the TSS bitmap happens on
+ * exit to user mode. So this needs no protection.
*/
- preempt_disable();
if (turn_on)
bitmap_clear(iobm->bitmap, from, num);
else
@@ -69,11 +67,8 @@ long ksys_ioperm(unsigned long from, unsigned long num, int turn_on)
max_long = i;
}

- bytes = (max_long + 1) * sizeof(unsigned long);
- bytes_updated = max(bytes, t->io_bitmap->max);
+ iobm->max = (max_long + 1) * sizeof(unsigned long);

- /* Update the thread data */
- iobm->max = bytes;
/* Update the sequence number to force an update in switch_to() */
iobm->sequence = atomic64_add_return(1, &io_bitmap_sequence);

@@ -85,18 +80,6 @@ long ksys_ioperm(unsigned long from, unsigned long num, int turn_on)
t->io_bitmap = iobm;
set_thread_flag(TIF_IO_BITMAP);

- /* Update the TSS */
- tss = this_cpu_ptr(&cpu_tss_rw);
- memcpy(tss->io_bitmap.bitmap, iobm->bitmap, bytes_updated);
- /* Store the new end of the zero bits */
- tss->io_bitmap.prev_max = bytes;
- /* Make the bitmap base in the TSS valid */
- tss->x86_tss.io_bitmap_base = IO_BITMAP_OFFSET_VALID;
- /* Make sure the TSS limit covers the I/O bitmap. */
- refresh_tss_limit();
-
- preempt_enable();
-
return 0;
}

diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
index eec7987..1990011 100644
--- a/arch/x86/kernel/process.c
+++ b/arch/x86/kernel/process.c
@@ -354,8 +354,34 @@ void arch_setup_new_exec(void)
}
}

-static void switch_to_update_io_bitmap(struct tss_struct *tss,
- struct io_bitmap *iobm)
+static inline void tss_invalidate_io_bitmap(struct tss_struct *tss)
+{
+ /*
+ * Invalidate the I/O bitmap by moving io_bitmap_base outside the
+ * TSS limit so any subsequent I/O access from user space will
+ * trigger a #GP.
+ *
+ * This is correct even when VMEXIT rewrites the TSS limit
+ * to 0x67 as the only requirement is that the base points
+ * outside the limit.
+ */
+ tss->x86_tss.io_bitmap_base = IO_BITMAP_OFFSET_INVALID;
+}
+
+static inline void switch_to_bitmap(unsigned long tifp)
+{
+ /*
+ * Invalidate I/O bitmap if the previous task used it. This prevents
+ * any possible leakage of an active I/O bitmap.
+ *
+ * If the next task has an I/O bitmap it will handle it on exit to
+ * user mode.
+ */
+ if (tifp & _TIF_IO_BITMAP)
+ tss_invalidate_io_bitmap(this_cpu_ptr(&cpu_tss_rw));
+}
+
+static void tss_copy_io_bitmap(struct tss_struct *tss, struct io_bitmap *iobm)
{
/*
* Copy at least the byte range of the incoming tasks bitmap which
@@ -376,13 +402,15 @@ static void switch_to_update_io_bitmap(struct tss_struct *tss,
tss->io_bitmap.prev_sequence = iobm->sequence;
}

-static inline void switch_to_bitmap(struct thread_struct *next,
- unsigned long tifp, unsigned long tifn)
+/**
+ * tss_update_io_bitmap - Update I/O bitmap before exiting to usermode
+ */
+void tss_update_io_bitmap(void)
{
struct tss_struct *tss = this_cpu_ptr(&cpu_tss_rw);

- if (tifn & _TIF_IO_BITMAP) {
- struct io_bitmap *iobm = next->io_bitmap;
+ if (test_thread_flag(TIF_IO_BITMAP)) {
+ struct io_bitmap *iobm = current->thread.io_bitmap;

/*
* Only copy bitmap data when the sequence number
@@ -390,7 +418,7 @@ static inline void switch_to_bitmap(struct thread_struct *next,
* task.
*/
if (tss->io_bitmap.prev_sequence != iobm->sequence)
- switch_to_update_io_bitmap(tss, iobm);
+ tss_copy_io_bitmap(tss, iobm);

/* Enable the bitmap */
tss->x86_tss.io_bitmap_base = IO_BITMAP_OFFSET_VALID;
@@ -403,18 +431,8 @@ static inline void switch_to_bitmap(struct thread_struct *next,
* limit.
*/
refresh_tss_limit();
- } else if (tifp & _TIF_IO_BITMAP) {
- /*
- * Do not touch the bitmap. Let the next bitmap using task
- * deal with the mess. Just make the io_bitmap_base invalid
- * by moving it outside the TSS limit so any subsequent I/O
- * access from user space will trigger a #GP.
- *
- * This is correct even when VMEXIT rewrites the TSS limit
- * to 0x67 as the only requirement is that the base points
- * outside the limit.
- */
- tss->x86_tss.io_bitmap_base = IO_BITMAP_OFFSET_INVALID;
+ } else {
+ tss_invalidate_io_bitmap(tss);
}
}

@@ -628,7 +646,8 @@ void __switch_to_xtra(struct task_struct *prev_p, struct task_struct *next_p)

tifn = READ_ONCE(task_thread_info(next_p)->flags);
tifp = READ_ONCE(task_thread_info(prev_p)->flags);
- switch_to_bitmap(next, tifp, tifn);
+
+ switch_to_bitmap(tifp);

propagate_user_return_notify(prev_p, next_p);

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