[efi] Run at TPL_CALLBACK to protect against UEFI timers

As noted in the comments, UEFI manages to combines the all of the
worst aspects of both a polling design (inefficiency and inability to
sleep until something interesting happens) and of an interrupt-driven
design (the complexity of code that could be preempted at any time,
thanks to UEFI timers).

This causes problems in particular for UEFI USB keyboards: the
keyboard driver calls UsbAsyncInterruptTransfer() to set up a periodic
timer which is used to poll the USB bus.  This poll may interrupt a
critical section within iPXE, typically resulting in list corruption
and either a hang or reboot.

Work around this problem by mirroring the BIOS design, in which we run
with interrupts disabled almost all of the time.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

1 files changed, 12 insertions, 0 deletions

diff --git a/src/interface/efi/efi_snp.c b/src/interface/efi/efi_snp.c
index 263a25ac6..88d999bf0 100644
--- a/src/interface/efi/efi_snp.c
+++ b/src/interface/efi/efi_snp.c
@@ -45,6 +45,9 @@ static LIST_HEAD ( efi_snp_devices );
 /** Network devices are currently claimed for use by iPXE */
 static int efi_snp_claimed;
 
+/** TPL prior to network devices being claimed */
+static EFI_TPL efi_snp_old_tpl;
+
 /* Downgrade user experience if configured to do so
  *
  * The default UEFI user experience for network boot is somewhat
@@ -1895,8 +1898,13 @@ struct efi_snp_device * last_opened_snpdev ( void ) {
  * @v delta		Claim count change
  */
 void efi_snp_add_claim ( int delta ) {
+	EFI_BOOT_SERVICES *bs = efi_systab->BootServices;
 	struct efi_snp_device *snpdev;
 
+	/* Raise TPL if we are about to claim devices */
+	if ( ! efi_snp_claimed )
+		efi_snp_old_tpl = bs->RaiseTPL ( TPL_CALLBACK );
+
 	/* Claim SNP devices */
 	efi_snp_claimed += delta;
 	assert ( efi_snp_claimed >= 0 );
@@ -1904,4 +1912,8 @@ void efi_snp_add_claim ( int delta ) {
 	/* Update SNP mode state for each interface */
 	list_for_each_entry ( snpdev, &efi_snp_devices, list )
 		efi_snp_set_state ( snpdev );
+
+	/* Restore TPL if we have released devices */
+	if ( ! efi_snp_claimed )
+		bs->RestoreTPL ( efi_snp_old_tpl );
 }