xen/arm: Check if the FDT passed by the bootloader is valid

There is currently no sanity check on the FDT passed by the bootloader.
Whilst they are stricly not necessary, it will avoid us to spend hours
to try to find out why it does not work.

>From the booting documentation for AArch32 [1] and AArch64 [2] must :
    - be placed on 8-byte boundary
    - not exceed 2MB (only on AArch64)

Even if AArch32 does not seem to limit the size, Xen is not currently
able to support more the 2MB FDT. It is better to crash rather with a nice
error message than claiming we are supporting any size of FDT.

The checks are mostly borrowed from the Linux code (see fixmap_remap_fdt
in arch/arm64/mm/mmu.c).

[1] Section 2 in linux/Documentation/arm64/booting.txt
[2] Section 4b in linux/Documentation/arm/Booting

Signed-off-by: Julien Grall <julien.grall@arm.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
(cherry picked from commit 4a9d402746fd6fbd1f319eff29fa768f7d7aa62b)

diff --git a/xen/arch/arm/mm.c b/xen/arch/arm/mm.c
index f6d75559f7edaf23a7daf49f521752866c0b157d..a0c1e1269c6df7b6c5a2557380308de7f86e3c57 100644 (file)
--- a/xen/arch/arm/mm.c
+++ b/xen/arch/arm/mm.c
@@ -40,6 +40,8 @@
 #include <xsm/xsm.h>
 #include <xen/pfn.h>
 #include <xen/sizes.h>
+#include <xen/libfdt/libfdt.h>
+#include <asm/setup.h>
 
 struct domain *dom_xen, *dom_io, *dom_cow;
 
@@ -475,11 +477,36 @@ void * __init early_fdt_map(paddr_t fdt_paddr)
 {
     /* We are using 2MB superpage for mapping the FDT */
     paddr_t base_paddr = fdt_paddr & SECOND_MASK;
+    paddr_t offset;
+    void *fdt_virt;
+
+    /*
+     * Check whether the physical FDT address is set and meets the minimum
+     * alignment requirement. Since we are relying on MIN_FDT_ALIGN to be at
+     * least 8 bytes so that we always access the magic and size fields
+     * of the FDT header after mapping the first chunk, double check if
+     * that is indeed the case.
+     */
+    BUILD_BUG_ON(MIN_FDT_ALIGN < 8);
+    if ( !fdt_paddr || fdt_paddr % MIN_FDT_ALIGN )
+        return NULL;
+
+    /* The FDT is mapped using 2MB superpage */
+    BUILD_BUG_ON(BOOT_FDT_VIRT_START % SZ_2M);
 
     create_mappings(boot_second, BOOT_FDT_VIRT_START, paddr_to_pfn(base_paddr),
                     SZ_2M >> PAGE_SHIFT, SZ_2M);
 
-    return (void *)BOOT_FDT_VIRT_START + (fdt_paddr % SECOND_SIZE);
+    offset = fdt_paddr % SECOND_SIZE;
+    fdt_virt = (void *)BOOT_FDT_VIRT_START + offset;
+
+    if ( fdt_magic(fdt_virt) != FDT_MAGIC )
+        return NULL;
+
+    if ( fdt_totalsize(fdt_virt) > MAX_FDT_SIZE )
+        return NULL;
+
+    return fdt_virt;
 }
 
 void __init remove_early_mappings(void)

diff --git a/xen/arch/arm/setup.c b/xen/arch/arm/setup.c
index 3a99ea847a417800c52918c09f7a674944c1c4c0..f4e9605bdc23e69d22ae982f86bc78f49e2ffecd 100644 (file)
--- a/xen/arch/arm/setup.c
+++ b/xen/arch/arm/setup.c
@@ -733,6 +733,12 @@ void __init start_xen(unsigned long boot_phys_offset,
     smp_clear_cpu_maps();
 
     device_tree_flattened = early_fdt_map(fdt_paddr);
+    if ( !device_tree_flattened )
+        panic("Invalid device tree blob at physical address %#lx.\n"
+              "The DTB must be 8-byte aligned and must not exceed 2 MB in size.\n\n"
+              "Please check your bootloader.",
+              fdt_paddr);
+
     fdt_size = boot_fdt_info(device_tree_flattened, fdt_paddr);
 
     cmdline = boot_fdt_cmdline(device_tree_flattened);

diff --git a/xen/include/asm-arm/setup.h b/xen/include/asm-arm/setup.h
index 3da7a4904af450d5a7fec9b1477e6cf959fb7c5d..2efa6cbd99ca9e78e816ab66ffdf55f57d9ca07c 100644 (file)
--- a/xen/include/asm-arm/setup.h
+++ b/xen/include/asm-arm/setup.h
@@ -3,6 +3,9 @@
 
 #include <public/version.h>
 
+#define MIN_FDT_ALIGN 8
+#define MAX_FDT_SIZE SZ_2M
+
 #define NR_MEM_BANKS 64
 
 #define MAX_MODULES 5 /* Current maximum useful modules */