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efi: Allow disabling PCI busmastering on bridges during boot

PATCH In review Ard Biesheuvel 2 participants 2 messages Dec 31, 2019 – Jan 3, 2020

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Ard Biesheuvel

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PATCH v3 1/1 efi: Allow disabling PCI busmastering on bridges during boot

From: Matthew Garrett <[email protected]>

Add an option to disable the busmaster bit in the control register on all PCI bridges calling ExitBootServices() and passing control to the runtime kernel. System firmware may configure the IOMMU to prevent malicious PCI devices from being able to attack the OS via DMA. However, since firmware can't guarantee that the OS is IOMMU-aware, it will tear down IOMMU configuration when ExitBootServices() is called. This leaves a window between where a hostile device could still cause damage before Linux configures the IOMMU again.

If CONFIG_EFI_DISABLE_PCI_DMA is enabled or "efi=disable_early_pci_dma" is passed on the command line, the EFI stub will clear the busmaster bit on all PCI bridges before ExitBootServices() is called. This will prevent any malicious PCI devices from being able to perform DMA until the kernel reenables busmastering after configuring the IOMMU.

This option may cause failures with some poorly behaved hardware and should not be enabled without testing. The kernel commandline options "efi=disable_early_pci_dma" or "efi=no_disable_early_pci_dma" may be used to override the default. Note that PCI devices downstream from PCI bridges are disconnected from their drivers first, using the UEFI driver model API, so that DMA can be disabled safely at the bridge level.

Cc Andy Lutomirski <[email protected]>
Cc Arvind Sankar <[email protected]>
Co-developed-by Matthew Garrett <[email protected]>
Signed-off-by Matthew Garrett <[email protected]>
[ardb: disconnect PCI I/O handles first, as suggested by Arvind]
Signed-off-by Ard Biesheuvel <[email protected]>
---
 Documentation/admin-guide/kernel-parameters.txt |   4 +
 arch/x86/boot/compressed/eboot.c                |   2 +
 drivers/firmware/efi/Kconfig                    |  22 ++++
 drivers/firmware/efi/libstub/Makefile           |   2 +-
 drivers/firmware/efi/libstub/arm-stub.c         |   1 +
 drivers/firmware/efi/libstub/efi-stub-helper.c  |  17 +++
 drivers/firmware/efi/libstub/efistub.h          |   1 +
 drivers/firmware/efi/libstub/pci.c              | 118 ++++++++++++++++++++
 include/linux/efi.h                             |   6 +-
 9 files changed, 171 insertions(+), 2 deletions(-)
Documentation/admin-guide/kernel-parameters.txt +4 -0
index fbcf8d1eb3ee..9db18aa1f75c 100644
@@ -1177,6 +1177,10 @@
 			claim. Specify efi=nosoftreserve to disable this
 			reservation and treat the memory by its base type
 			(i.e. EFI_CONVENTIONAL_MEMORY / "System RAM").
+			disable_early_pci_dma: Disable the busmaster bit on all
+			PCI bridges while in the EFI boot stub
+			no_disable_early_pci_dma: Leave the busmaster bit set
+			on all PCI bridges while in the EFI boot stub
 
 	efi_no_storage_paranoia [EFI; X86]
 			Using this parameter you can use more than 50% of
arch/x86/boot/compressed/eboot.c +2 -0
index 98477f3529f6..4a96761a2bfd 100644
@@ -166,6 +166,8 @@ static void setup_efi_pci(struct boot_params *params)
 
 free_handle:
 	efi_bs_call(free_pool, pci_handle);
+
+	efi_pci_disable_bridge_busmaster();
 }
 
 static void retrieve_apple_device_properties(struct boot_params *boot_params)
drivers/firmware/efi/Kconfig +22 -0
index bcc378c19ebe..5e78c5f6c751 100644
@@ -215,6 +215,28 @@ config EFI_RCI2_TABLE
 
 	  Say Y here for Dell EMC PowerEdge systems.
 
+config EFI_DISABLE_PCI_DMA
+       bool "Clear Busmaster bit on PCI bridges during ExitBootServices()"
+       help
+	  Disable the busmaster bit in the control register on all PCI bridges
+	  while calling ExitBootServices() and passing control to the runtime
+	  kernel. System firmware may configure the IOMMU to prevent malicious
+	  PCI devices from being able to attack the OS via DMA. However, since
+	  firmware can't guarantee that the OS is IOMMU-aware, it will tear
+	  down IOMMU configuration when ExitBootServices() is called. This
+	  leaves a window between where a hostile device could still cause
+	  damage before Linux configures the IOMMU again.
+
+	  If you say Y here, the EFI stub will clear the busmaster bit on all
+	  PCI bridges before ExitBootServices() is called. This will prevent
+	  any malicious PCI devices from being able to perform DMA until the
+	  kernel reenables busmastering after configuring the IOMMU.
+
+	  This option will cause failures with some poorly behaved hardware
+	  and should not be enabled without testing. The kernel commandline
+	  options "efi=disable_early_pci_dma" or "efi=no_disable_early_pci_dma"
+	  may be used to override this option.
+
 endmenu
 
 config UEFI_CPER
drivers/firmware/efi/libstub/Makefile +1 -1
index c35f893897e1..98a81576213d 100644
@@ -39,7 +39,7 @@ OBJECT_FILES_NON_STANDARD	:= y
 KCOV_INSTRUMENT			:= n
 
 lib-y				:= efi-stub-helper.o gop.o secureboot.o tpm.o \
-				   random.o
+				   random.o pci.o
 
 # include the stub's generic dependencies from lib/ when building for ARM/arm64
 arm-deps-y := fdt_rw.c fdt_ro.c fdt_wip.c fdt.c fdt_empty_tree.c fdt_sw.c
drivers/firmware/efi/libstub/arm-stub.c +1 -0
index 7bbef4a67350..9857f831638c 100644
@@ -256,6 +256,7 @@ unsigned long efi_entry(void *handle, efi_system_table_t *sys_table_arg,
 	}
 
 	install_memreserve_table();
+	efi_pci_disable_bridge_busmaster();
 
 	new_fdt_addr = fdt_addr;
 	status = allocate_new_fdt_and_exit_boot(handle,
drivers/firmware/efi/libstub/efi-stub-helper.c +17 -0
index f1b9c36934e9..47dc404675b2 100644
@@ -33,6 +33,8 @@ static bool __efistub_global efi_nokaslr;
 static bool __efistub_global efi_quiet;
 static bool __efistub_global efi_novamap;
 static bool __efistub_global efi_nosoftreserve;
+static bool __efistub_global efi_disable_pci_dma =
+					IS_ENABLED(CONFIG_EFI_DISABLE_PCI_DMA);
 
 bool __pure nokaslr(void)
 {

@@ -51,6 +53,11 @@ bool __pure __efi_soft_reserve_enabled(void)
 	return !efi_nosoftreserve;
 }
 
+bool __pure disable_pci_dma(void)
+{
+	return efi_disable_pci_dma;
+}
+
 #define EFI_MMAP_NR_SLACK_SLOTS	8
 
 struct file_info {

@@ -493,6 +500,16 @@ efi_status_t efi_parse_options(char const *cmdline)
 			efi_nosoftreserve = true;
 		}
 
+		if (!strncmp(str, "disable_early_pci_dma", 21)) {
+			str += strlen("disable_early_pci_dma");
+			efi_disable_pci_dma = true;
+		}
+
+		if (!strncmp(str, "no_disable_early_pci_dma", 24)) {
+			str += strlen("no_disable_early_pci_dma");
+			efi_disable_pci_dma = false;
+		}
+
 		/* Group words together, delimited by "," */
 		while (*str && *str != ' ' && *str != ',')
 			str++;
drivers/firmware/efi/libstub/efistub.h +1 -0
index c244b165005e..2748eb4c7831 100644
@@ -34,6 +34,7 @@
 extern bool __pure nokaslr(void);
 extern bool __pure is_quiet(void);
 extern bool __pure novamap(void);
+extern bool __pure disable_pci_dma(void);
 
 extern __pure efi_system_table_t  *efi_system_table(void);
drivers/firmware/efi/libstub/pci.c new file +118 -0

118 lines not on this page · raw message

include/linux/efi.h +5 -1
index ee68ea6f85ff..7e8e25b1d11c 100644
@@ -319,7 +319,9 @@ typedef union {
 		void *stall;
 		void *set_watchdog_timer;
 		void *connect_controller;
-		void *disconnect_controller;
+		efi_status_t (__efiapi *disconnect_controller)(efi_handle_t,
+							       efi_handle_t,
+							       efi_handle_t);
 		void *open_protocol;
 		void *close_protocol;
 		void *open_protocol_information;

@@ -1692,4 +1694,6 @@ struct linux_efi_memreserve {
 #define EFI_MEMRESERVE_COUNT(size) (((size) - sizeof(struct linux_efi_memreserve)) \
 	/ sizeof(((struct linux_efi_memreserve *)0)->entry[0]))
 
+void efi_pci_disable_bridge_busmaster(void);
+
 #endif /* _LINUX_EFI_H */
Signature · 1 line
2.20.1