PCI: Block on access to temporarily unavailable pci device

The existing implementation of pci_block_user_cfg_access() was recently
criticised for providing out of date information and for returning errors
on write, which applications won't be expecting.

This reimplementation uses a global wait queue and a bit per device.
I've open-coded prepare_to_wait() / finish_wait() as I could optimise
it significantly by knowing that the pci_lock protected us at all points.

It looked a bit funny to be doing a spin_unlock_irqsave(); schedule(),
so I used spin_lock_irq() for the _user versions of pci_read_config and
pci_write_config.  Not carrying a flags pointer around made the code
much less nasty.

Attempts to block an already blocked device hit a BUG() and attempts to
unblock an already unblocked device hit a WARN().  If we need to block
access to a device from userspace, it's because it's unsafe for even
another bit of the kernel to access the device.  An attempt to block
a device for a second time means we're about to access the device to
perform some other operation, which could provoke undefined behaviour
from the device.

Signed-off-by: Matthew Wilcox <matthew@wil.cx>
Acked-by: Adam Belay <abelay@novell.com>
Acked-by: Alan Cox <alan@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

1 files changed, 47 insertions, 28 deletions

diff --git a/drivers/pci/access.c b/drivers/pci/access.c
index ea16805a153c..73a58c73d526 100644
--- a/drivers/pci/access.c
+++ b/drivers/pci/access.c
@@ -1,6 +1,7 @@
 #include <linux/pci.h>
 #include <linux/module.h>
 #include <linux/ioport.h>
+#include <linux/wait.h>
 
 #include "pci.h"
 
@@ -63,30 +64,42 @@ EXPORT_SYMBOL(pci_bus_write_config_byte);
 EXPORT_SYMBOL(pci_bus_write_config_word);
 EXPORT_SYMBOL(pci_bus_write_config_dword);
 
-static u32 pci_user_cached_config(struct pci_dev *dev, int pos)
-{
-        u32 data;
+/*
+ * The following routines are to prevent the user from accessing PCI config
+ * space when it's unsafe to do so.  Some devices require this during BIST and
+ * we're required to prevent it during D-state transitions.
+ *
+ * We have a bit per device to indicate it's blocked and a global wait queue
+ * for callers to sleep on until devices are unblocked.
+ */
+static DECLARE_WAIT_QUEUE_HEAD(pci_ucfg_wait);
 
-        data = dev->saved_config_space[pos/sizeof(dev->saved_config_space[0])];
-        data >>= (pos % sizeof(dev->saved_config_space[0])) * 8;
-        return data;
+static noinline void pci_wait_ucfg(struct pci_dev *dev)
+{
+        DECLARE_WAITQUEUE(wait, current);
+
+        __add_wait_queue(&pci_ucfg_wait, &wait);
+        do {
+                set_current_state(TASK_UNINTERRUPTIBLE);
+                spin_unlock_irq(&pci_lock);
+                schedule();
+                spin_lock_irq(&pci_lock);
+        } while (dev->block_ucfg_access);
+        __remove_wait_queue(&pci_ucfg_wait, &wait);
 }
 
 #define PCI_USER_READ_CONFIG(size,type)                                 \
 int pci_user_read_config_##size                                         \
         (struct pci_dev *dev, int pos, type *val)                       \
 {                                                                       \
-        unsigned long flags;                                            \
         int ret = 0;                                                    \
         u32 data = -1;                                                  \
         if (PCI_##size##_BAD) return PCIBIOS_BAD_REGISTER_NUMBER;       \
-        spin_lock_irqsave(&pci_lock, flags);                            \
-        if (likely(!dev->block_ucfg_access))                            \
-                ret = dev->bus->ops->read(dev->bus, dev->devfn,         \
+        spin_lock_irq(&pci_lock);                                       \
+        if (unlikely(dev->block_ucfg_access)) pci_wait_ucfg(dev);       \
+        ret = dev->bus->ops->read(dev->bus, dev->devfn,                 \
                                         pos, sizeof(type), &data);      \
-        else if (pos < sizeof(dev->saved_config_space))                 \
-                data = pci_user_cached_config(dev, pos);                \
-        spin_unlock_irqrestore(&pci_lock, flags);                       \
+        spin_unlock_irq(&pci_lock);                                     \
         *val = (type)data;                                              \
         return ret;                                                     \
 }
@@ -95,14 +108,13 @@ int pci_user_read_config_##size						\
 int pci_user_write_config_##size                                        \
         (struct pci_dev *dev, int pos, type val)                        \
 {                                                                       \
-        unsigned long flags;                                            \
         int ret = -EIO;                                                 \
         if (PCI_##size##_BAD) return PCIBIOS_BAD_REGISTER_NUMBER;       \
-        spin_lock_irqsave(&pci_lock, flags);                            \
-        if (likely(!dev->block_ucfg_access))                            \
-                ret = dev->bus->ops->write(dev->bus, dev->devfn,        \
+        spin_lock_irq(&pci_lock);                                       \
+        if (unlikely(dev->block_ucfg_access)) pci_wait_ucfg(dev);       \
+        ret = dev->bus->ops->write(dev->bus, dev->devfn,                \
                                         pos, sizeof(type), val);        \
-        spin_unlock_irqrestore(&pci_lock, flags);                       \
+        spin_unlock_irq(&pci_lock);                                     \
         return ret;                                                     \
 }
 
@@ -117,21 +129,23 @@ PCI_USER_WRITE_CONFIG(dword, u32)
  * pci_block_user_cfg_access - Block userspace PCI config reads/writes
  * @dev:        pci device struct
  *
- * This function blocks any userspace PCI config accesses from occurring.
- * When blocked, any writes will be bit bucketed and reads will return the
- * data saved using pci_save_state for the first 64 bytes of config
- * space and return 0xff for all other config reads.
- **/
+ * When user access is blocked, any reads or writes to config space will
+ * sleep until access is unblocked again.  We don't allow nesting of
+ * block/unblock calls.
+ */
 void pci_block_user_cfg_access(struct pci_dev *dev)
 {
         unsigned long flags;
+        int was_blocked;
 
-        pci_save_state(dev);
-
-        /* spinlock to synchronize with anyone reading config space now */
         spin_lock_irqsave(&pci_lock, flags);
+        was_blocked = dev->block_ucfg_access;
         dev->block_ucfg_access = 1;
         spin_unlock_irqrestore(&pci_lock, flags);
+
+        /* If we BUG() inside the pci_lock, we're guaranteed to hose
+         * the machine */
+        BUG_ON(was_blocked);
 }
 EXPORT_SYMBOL_GPL(pci_block_user_cfg_access);
 
@@ -140,14 +154,19 @@ EXPORT_SYMBOL_GPL(pci_block_user_cfg_access);
  * @dev:        pci device struct
  *
  * This function allows userspace PCI config accesses to resume.
- **/
+ */
 void pci_unblock_user_cfg_access(struct pci_dev *dev)
 {
         unsigned long flags;
 
-        /* spinlock to synchronize with anyone reading saved config space */
         spin_lock_irqsave(&pci_lock, flags);
+
+        /* This indicates a problem in the caller, but we don't need
+         * to kill them, unlike a double-block above. */
+        WARN_ON(!dev->block_ucfg_access);
+
         dev->block_ucfg_access = 0;
+        wake_up_all(&pci_ucfg_wait);
         spin_unlock_irqrestore(&pci_lock, flags);
 }
 EXPORT_SYMBOL_GPL(pci_unblock_user_cfg_access);