[PATCH v3 0/4] efi/arm64: work around Image placement issues

[PATCH v3 0/4] efi/arm64: work around Image placement issues

[Ard Biesheuvel]

Ben reported that distro GRUB may fail to boot in some circumstances,
and tracked it down to an issue in the way distro GRUB allocates space
for the image. Due to an oversight (addressed in patch #2), this
condition is rarely triggered, but let's work around it in any case (#1)

Remaining patches add further warnings for conditions that are unlikely
to occur, but should not be ignored.

Build tested only.

Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>

Ard Biesheuvel (4):
  efi/libstub: arm64: Force Image reallocation if BSS was not reserved
  efi/libstub: arm64: Relax 2M alignment again for relocatable kernels
  efi/libstub: arm64: Warn when efi_random_alloc() fails
  efi/libstub: arm64: Double check image alignment at entry

 drivers/firmware/efi/libstub/arm64-stub.c | 71 +++++++++++++++++---
 1 file changed, 61 insertions(+), 10 deletions(-)

-- 
2.20.1

[PATCH v3 1/4] efi/libstub: arm64: Force Image reallocation if BSS was not reserved

[Ard Biesheuvel]

Distro versions of GRUB replace the usual LoadImage/StartImage calls
used to load the kernel image with some local code that fails to honor
the allocation requirements described in the PE/COFF header, as it
does not account for the image's BSS section at all: it fails to
allocate space for it, and fails to zero initialize it.

Since the EFI stub itself is allocated in the .init segment, which is
in the middle of the image, its BSS section is not impacted by this,
and the main consequence of this omission is that the BSS section may
overlap with memory regions that are already used by the firmware.

So let's warn about this condition, and force image reallocation to
occur in this case, which works around the problem.

Fixes: 82046702e288 ("efi/libstub/arm64: Replace 'preferred' offset with alignment check")
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
---
 drivers/firmware/efi/libstub/arm64-stub.c | 49 +++++++++++++++++++-
 1 file changed, 48 insertions(+), 1 deletion(-)

diff --git a/drivers/firmware/efi/libstub/arm64-stub.c b/drivers/firmware/efi/libstub/arm64-stub.c
index 7bf0a7acae5e..3698c1ce2940 100644
--- a/drivers/firmware/efi/libstub/arm64-stub.c
+++ b/drivers/firmware/efi/libstub/arm64-stub.c
@@ -34,6 +34,51 @@ efi_status_t check_platform_features(void)
 	return EFI_SUCCESS;
 }
 
+/*
+ * Distro versions of GRUB may ignore the BSS allocation entirely (i.e., fail
+ * to provide space, and fail to zero it). Check for this condition by double
+ * checking that the first and the last byte of the image are covered by the
+ * same EFI memory map entry.
+ */
+static bool check_image_region(u64 base, u64 size)
+{
+	unsigned long map_size, desc_size, buff_size;
+	efi_memory_desc_t *memory_map;
+	struct efi_boot_memmap map;
+	efi_status_t status;
+	bool ret = false;
+	int map_offset;
+
+	map.map =	&memory_map;
+	map.map_size =	&map_size;
+	map.desc_size =	&desc_size;
+	map.desc_ver =	NULL;
+	map.key_ptr =	NULL;
+	map.buff_size =	&buff_size;
+
+	status = efi_get_memory_map(&map);
+	if (status != EFI_SUCCESS)
+		return false;
+
+	for (map_offset = 0; map_offset < map_size; map_offset += desc_size) {
+		efi_memory_desc_t *md = (void *)memory_map + map_offset;
+		u64 end = md->phys_addr + md->num_pages * EFI_PAGE_SIZE;
+
+		/*
+		 * Find the region that covers base, and return whether
+		 * it covers base+size bytes.
+		 */
+		if (base >= md->phys_addr && base < end) {
+			ret = (base + size) <= end;
+			break;
+		}
+	}
+
+	efi_bs_call(free_pool, memory_map);
+
+	return ret;
+}
+
 /*
  * Although relocatable kernels can fix up the misalignment with respect to
  * MIN_KIMG_ALIGN, the resulting virtual text addresses are subtly out of
@@ -92,7 +137,9 @@ efi_status_t handle_kernel_image(unsigned long *image_addr,
 	}
 
 	if (status != EFI_SUCCESS) {
-		if (IS_ALIGNED((u64)_text, min_kimg_align())) {
+		if (!check_image_region((u64)_text, kernel_memsize)) {
+			efi_err("FIRMWARE BUG: Image BSS overlaps adjacent EFI memory region\n");
+		} else if (IS_ALIGNED((u64)_text, min_kimg_align())) {
 			/*
 			 * Just execute from wherever we were loaded by the
 			 * UEFI PE/COFF loader if the alignment is suitable.
-- 
2.20.1

[PATCH v3 2/4] efi/libstub: arm64: Relax 2M alignment again for relocatable kernels

[Ard Biesheuvel]

Commit 82046702e288 ("efi/libstub/arm64: Replace 'preferred' offset with
alignment check") simplified the way the stub moves the kernel image
around in memory before booting it, given that a relocatable image does
not need to be copied to a 2M aligned offset if it was loaded on a 64k
boundary by EFI.

Commit d32de9130f6c ("efi/arm64: libstub: Deal gracefully with
EFI_RNG_PROTOCOL failure") inadvertently defeated this logic by
overriding the value of efi_nokaslr if EFI_RNG_PROTOCOL is not
available, which was mistaked by the loader logic as an explicit request
on the part of the user to disable KASLR and any associated relocation
of an Image not loaded on a 2M boundary.

So let's reinstate this functionality, by capturing the value of
efi_nokaslr at function entry to choose the minimum alignment.

Fixes: d32de9130f6c ("efi/arm64: libstub: Deal gracefully with EFI_RNG_PROTOCOL failure")
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
---
 drivers/firmware/efi/libstub/arm64-stub.c | 28 +++++++++-----------
 1 file changed, 13 insertions(+), 15 deletions(-)

diff --git a/drivers/firmware/efi/libstub/arm64-stub.c b/drivers/firmware/efi/libstub/arm64-stub.c
index 3698c1ce2940..6f214c9c303e 100644
--- a/drivers/firmware/efi/libstub/arm64-stub.c
+++ b/drivers/firmware/efi/libstub/arm64-stub.c
@@ -79,18 +79,6 @@ static bool check_image_region(u64 base, u64 size)
 	return ret;
 }
 
-/*
- * Although relocatable kernels can fix up the misalignment with respect to
- * MIN_KIMG_ALIGN, the resulting virtual text addresses are subtly out of
- * sync with those recorded in the vmlinux when kaslr is disabled but the
- * image required relocation anyway. Therefore retain 2M alignment unless
- * KASLR is in use.
- */
-static u64 min_kimg_align(void)
-{
-	return efi_nokaslr ? MIN_KIMG_ALIGN : EFI_KIMG_ALIGN;
-}
-
 efi_status_t handle_kernel_image(unsigned long *image_addr,
 				 unsigned long *image_size,
 				 unsigned long *reserve_addr,
@@ -101,6 +89,16 @@ efi_status_t handle_kernel_image(unsigned long *image_addr,
 	unsigned long kernel_size, kernel_memsize = 0;
 	u32 phys_seed = 0;
 
+	/*
+	 * Although relocatable kernels can fix up the misalignment with
+	 * respect to MIN_KIMG_ALIGN, the resulting virtual text addresses are
+	 * subtly out of sync with those recorded in the vmlinux when kaslr is
+	 * disabled but the image required relocation anyway. Therefore retain
+	 * 2M alignment if KASLR was explicitly disabled, even if it was not
+	 * going to be activated to begin with.
+	 */
+	u64 min_kimg_align = efi_nokaslr ? MIN_KIMG_ALIGN : EFI_KIMG_ALIGN;
+
 	if (IS_ENABLED(CONFIG_RANDOMIZE_BASE)) {
 		if (!efi_nokaslr) {
 			status = efi_get_random_bytes(sizeof(phys_seed),
@@ -130,7 +128,7 @@ efi_status_t handle_kernel_image(unsigned long *image_addr,
 		 * If KASLR is enabled, and we have some randomness available,
 		 * locate the kernel at a randomized offset in physical memory.
 		 */
-		status = efi_random_alloc(*reserve_size, min_kimg_align(),
+		status = efi_random_alloc(*reserve_size, min_kimg_align,
 					  reserve_addr, phys_seed);
 	} else {
 		status = EFI_OUT_OF_RESOURCES;
@@ -139,7 +137,7 @@ efi_status_t handle_kernel_image(unsigned long *image_addr,
 	if (status != EFI_SUCCESS) {
 		if (!check_image_region((u64)_text, kernel_memsize)) {
 			efi_err("FIRMWARE BUG: Image BSS overlaps adjacent EFI memory region\n");
-		} else if (IS_ALIGNED((u64)_text, min_kimg_align())) {
+		} else if (IS_ALIGNED((u64)_text, min_kimg_align)) {
 			/*
 			 * Just execute from wherever we were loaded by the
 			 * UEFI PE/COFF loader if the alignment is suitable.
@@ -150,7 +148,7 @@ efi_status_t handle_kernel_image(unsigned long *image_addr,
 		}
 
 		status = efi_allocate_pages_aligned(*reserve_size, reserve_addr,
-						    ULONG_MAX, min_kimg_align());
+						    ULONG_MAX, min_kimg_align);
 
 		if (status != EFI_SUCCESS) {
 			efi_err("Failed to relocate kernel\n");
-- 
2.20.1

[PATCH v3 3/4] efi/libstub: arm64: Warn when efi_random_alloc() fails

[Ard Biesheuvel]

Randomization of the physical load address of the kernel image relies on
efi_random_alloc() returning successfully, and currently, we ignore any
failures and just carry on, using the ordinary, non-randomized page
allocator routine. This means we never find out if a failure occurs,
which could harm security, so let's at least warn about this condition.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
---
 drivers/firmware/efi/libstub/arm64-stub.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/drivers/firmware/efi/libstub/arm64-stub.c b/drivers/firmware/efi/libstub/arm64-stub.c
index 6f214c9c303e..010564f8bbc4 100644
--- a/drivers/firmware/efi/libstub/arm64-stub.c
+++ b/drivers/firmware/efi/libstub/arm64-stub.c
@@ -130,6 +130,8 @@ efi_status_t handle_kernel_image(unsigned long *image_addr,
 		 */
 		status = efi_random_alloc(*reserve_size, min_kimg_align,
 					  reserve_addr, phys_seed);
+		if (status != EFI_SUCCESS)
+			efi_warn("efi_random_alloc() failed: 0x%lx\n", status);
 	} else {
 		status = EFI_OUT_OF_RESOURCES;
 	}
-- 
2.20.1

[PATCH v3 4/4] efi/libstub: arm64: Double check image alignment at entry

[Ard Biesheuvel]

On arm64, the stub only moves the kernel image around in memory if
needed, which is typically only for KASLR, given that relocatable
kernels (which is the default) can run from any 64k aligned address,
which is also the minimum alignment communicated to EFI via the PE/COFF
header.

Unfortunately, some loaders appear to ignore this header, and load the
kernel at some arbitrary offset in memory. We can deal with this, but
let's check for this condition anyway, so non-compliant code can be
spotted and fixed.

Cc: <stable@vger.kernel.org> # v5.10+
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
---
 drivers/firmware/efi/libstub/arm64-stub.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/drivers/firmware/efi/libstub/arm64-stub.c b/drivers/firmware/efi/libstub/arm64-stub.c
index 010564f8bbc4..2363fee9211c 100644
--- a/drivers/firmware/efi/libstub/arm64-stub.c
+++ b/drivers/firmware/efi/libstub/arm64-stub.c
@@ -119,6 +119,10 @@ efi_status_t handle_kernel_image(unsigned long *image_addr,
 	if (image->image_base != _text)
 		efi_err("FIRMWARE BUG: efi_loaded_image_t::image_base has bogus value\n");
 
+	if (!IS_ALIGNED((u64)_text, EFI_KIMG_ALIGN))
+		efi_err("FIRMWARE BUG: kernel image not aligned on %ldk boundary\n",
+			EFI_KIMG_ALIGN >> 10);
+
 	kernel_size = _edata - _text;
 	kernel_memsize = kernel_size + (_end - _edata);
 	*reserve_size = kernel_memsize;
-- 
2.20.1

Re: [PATCH v3 0/4] efi/arm64: work around Image placement issues

[Benjamin Herrenschmidt]

On Mon, 2021-07-26 at 16:51 +0200, Ard Biesheuvel wrote:
> Ben reported that distro GRUB may fail to boot in some circumstances,
> and tracked it down to an issue in the way distro GRUB allocates space
> for the image. Due to an oversight (addressed in patch #2), this
> condition is rarely triggered, but let's work around it in any case (#1)
> 
> Remaining patches add further warnings for conditions that are unlikely
> to occur, but should not be ignored.
> 
> Build tested only.
> 
> Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>

All 4 patches:

Tested-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

I checked that it catches all of the errors, including reverting my fix
for randomalloc.c and verifying that it caught (and successfully worked
around) the original boot  crash.

Cheers,
Ben.

Re: [PATCH v3 0/4] efi/arm64: work around Image placement issues

[Ard Biesheuvel]

On Mon, 2 Aug 2021 at 13:26, Benjamin Herrenschmidt
<benh@kernel.crashing.org> wrote:
>
> On Mon, 2021-07-26 at 16:51 +0200, Ard Biesheuvel wrote:
> > Ben reported that distro GRUB may fail to boot in some circumstances,
> > and tracked it down to an issue in the way distro GRUB allocates space
> > for the image. Due to an oversight (addressed in patch #2), this
> > condition is rarely triggered, but let's work around it in any case (#1)
> >
> > Remaining patches add further warnings for conditions that are unlikely
> > to occur, but should not be ignored.
> >
> > Build tested only.
> >
> > Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
>
> All 4 patches:
>
> Tested-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
>
> I checked that it catches all of the errors, including reverting my fix
> for randomalloc.c and verifying that it caught (and successfully worked
> around) the original boot  crash.
>

Excellent. Thanks for tracking this down and getting it fixed properly.