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authorArd Biesheuvel <ardb@kernel.org>2020-02-18 12:34:05 +0100
committerArd Biesheuvel <ardb@kernel.org>2020-02-22 19:04:54 +0100
commitc7225494be79f8629c9166b106d6b1febf2a882f (patch)
tree42feaf1e3e703f56a1f3fedd8fb20d9196cbf277
parentbb6d3fb354c5ee8d6bde2d576eb7220ea09862b9 (diff)
efi/arm: Work around missing cache maintenance in decompressor handover
The EFI stub executes within the context of the zImage as it was loaded by the firmware, which means it is treated as an ordinary PE/COFF executable, which is loaded into memory, and cleaned to the PoU to ensure that it can be executed safely while the MMU and caches are on. When the EFI stub hands over to the decompressor, we clean the caches by set/way and disable the MMU and D-cache, to comply with the Linux boot protocol for ARM. However, cache maintenance by set/way is not sufficient to ensure that subsequent instruction fetches and data accesses done with the MMU off see the correct data. This means that proceeding as we do currently is not safe, especially since we also perform data accesses with the MMU off, from a literal pool as well as the stack. So let's kick this can down the road a bit, and jump into the relocated zImage before disabling the caches. This removes the requirement to perform any by-VA cache maintenance on the original PE/COFF executable, but it does require that the relocated zImage is cleaned to the PoC, which is currently not the case. This will be addressed in a subsequent patch. Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
-rw-r--r--arch/arm/boot/compressed/head.S20
1 files changed, 14 insertions, 6 deletions
diff --git a/arch/arm/boot/compressed/head.S b/arch/arm/boot/compressed/head.S
index 088b0a060876..39f7071d47c7 100644
--- a/arch/arm/boot/compressed/head.S
+++ b/arch/arm/boot/compressed/head.S
@@ -1461,6 +1461,17 @@ ENTRY(efi_stub_entry)
@ Preserve return value of efi_entry() in r4
mov r4, r0
bl cache_clean_flush
+
+ @ The PE/COFF loader might not have cleaned the code we are
+ @ running beyond the PoU, and so calling cache_off below from
+ @ inside the PE/COFF loader allocated region is unsafe. Let's
+ @ assume our own zImage relocation code did a better job, and
+ @ jump into its version of this routine before proceeding.
+ ldr r0, [sp] @ relocated zImage
+ ldr r1, .Ljmp
+ sub r1, r0, r1
+ mov pc, r1 @ no mode switch
+0:
bl cache_off
@ Set parameters for booting zImage according to boot protocol
@@ -1469,18 +1480,15 @@ ENTRY(efi_stub_entry)
mov r0, #0
mov r1, #0xFFFFFFFF
mov r2, r4
-
- @ Branch to (possibly) relocated zImage that is in [sp]
- ldr lr, [sp]
- ldr ip, =start_offset
- add lr, lr, ip
- mov pc, lr @ no mode switch
+ b __efi_start
efi_load_fail:
@ Return EFI_LOAD_ERROR to EFI firmware on error.
ldr r0, =0x80000001
ldmfd sp!, {ip, pc}
ENDPROC(efi_stub_entry)
+ .align 2
+.Ljmp: .long start - 0b
#endif
.align