xen/arm: Rework the way to compute dom0 DTB base address

If the DTB is loading right after the kernel, on some setup, Linux will
overwrite the DTB during the decompression step.

To be sure the DTB won't be overwritten by the decompression stage, load
the DTB near the end of the first memory bank and below 4Gib (if memory range is
greater).

Signed-off-by: Julien Grall <julien.grall@linaro.org>
Acked-by: Ian Campbell <ian.campbell@citrix.com>

3 files changed, 41 insertions, 6 deletions

diff --git a/xen/arch/arm/domain_build.c b/xen/arch/arm/domain_build.c
index b92c64bcd0..0423b5a654 100644
--- a/xen/arch/arm/domain_build.c
+++ b/xen/arch/arm/domain_build.c
@@ -477,6 +477,7 @@ static int prepare_dtb(struct domain *d, struct kernel_info *kinfo)
     void *fdt;
     int new_size;
     int ret;
+    paddr_t end;
 
     kinfo->unassigned_mem = dom0_mem;
 
@@ -502,17 +503,26 @@ static int prepare_dtb(struct domain *d, struct kernel_info *kinfo)
         goto err;
 
     /*
-     * Put the device tree at the beginning of the first bank.  It
-     * must be below 4 GiB.
+     * DTB must be load below 4GiB and far enough from linux (Linux uses
+     * the space after it to decompress)
+     * Load the DTB at the end of the first bank, while ensuring it is
+     * also below 4G
      */
-    kinfo->dtb_paddr = kinfo->mem.bank[0].start + 0x100;
-    if ( kinfo->dtb_paddr + fdt_totalsize(kinfo->fdt) > (1ull << 32) )
+    end = kinfo->mem.bank[0].start + kinfo->mem.bank[0].size;
+    end = MIN(1ull << 32, end);
+    kinfo->dtb_paddr = end - fdt_totalsize(kinfo->fdt);
+    /* Align the address to 2Mb. Linux only requires 4 byte alignment */
+    kinfo->dtb_paddr &= ~((2 << 20) - 1);
+
+    if ( fdt_totalsize(kinfo->fdt) > end )
     {
-        printk("Not enough memory below 4 GiB for the device tree.");
+        printk(XENLOG_ERR "Not enough memory in the first bank for "
+               "the device tree.");
         ret = -FDT_ERR_XEN(EINVAL);
         goto err;
     }
 
+
     return 0;
 
   err:
@@ -525,6 +535,9 @@ static void dtb_load(struct kernel_info *kinfo)
 {
     void * __user dtb_virt = (void * __user)(register_t)kinfo->dtb_paddr;
 
+    printk("Loading dom0 DTB to 0x%"PRIpaddr"-0x%"PRIpaddr"\n",
+           kinfo->dtb_paddr, kinfo->dtb_paddr + fdt_totalsize(kinfo->fdt));
+
     raw_copy_to_guest(dtb_virt, kinfo->fdt, fdt_totalsize(kinfo->fdt));
     xfree(kinfo->fdt);
 }
@@ -559,10 +572,12 @@ int construct_dom0(struct domain *d)
     if ( rc < 0 )
         return rc;
 
+    if ( kinfo.check_overlap )
+        kinfo.check_overlap(&kinfo);
+
     /* The following loads use the domain's p2m */
     p2m_load_VTTBR(d);
 
-    kinfo.dtb_paddr = kinfo.zimage.load_addr + kinfo.zimage.len;
     kernel_load(&kinfo);
     dtb_load(&kinfo);
 
diff --git a/xen/arch/arm/kernel.c b/xen/arch/arm/kernel.c
index aba5441066..3953f0f37e 100644
--- a/xen/arch/arm/kernel.c
+++ b/xen/arch/arm/kernel.c
@@ -12,6 +12,7 @@
 #include <xen/sched.h>
 #include <asm/byteorder.h>
 #include <asm/setup.h>
+#include <xen/libfdt/libfdt.h>
 
 #include "kernel.h"
 
@@ -64,6 +65,21 @@ void copy_from_paddr(void *dst, paddr_t paddr, unsigned long len, int attrindx)
     clear_fixmap(FIXMAP_MISC);
 }
 
+static void kernel_zimage_check_overlap(struct kernel_info *info)
+{
+    paddr_t zimage_start = info->zimage.load_addr;
+    paddr_t zimage_end = info->zimage.load_addr + info->zimage.len;
+    paddr_t dtb_start = info->dtb_paddr;
+    paddr_t dtb_end = info->dtb_paddr + fdt_totalsize(info->fdt);
+
+    if ( (dtb_start > zimage_end) || (dtb_end < zimage_start) )
+        return;
+
+    panic(XENLOG_ERR "The kernel(0x%"PRIpaddr"-0x%"PRIpaddr
+          ") is overlapping the DTB(0x%"PRIpaddr"-0x%"PRIpaddr")\n",
+          zimage_start, zimage_end, dtb_start, dtb_end);
+}
+
 static void kernel_zimage_load(struct kernel_info *info)
 {
     paddr_t load_addr = info->zimage.load_addr;
@@ -152,6 +168,7 @@ static int kernel_try_zimage_prepare(struct kernel_info *info,
 
     info->entry = info->zimage.load_addr;
     info->load = kernel_zimage_load;
+    info->check_overlap = kernel_zimage_check_overlap;
 
     return 0;
 }
@@ -197,6 +214,7 @@ static int kernel_try_elf_prepare(struct kernel_info *info,
      */
     info->entry = info->elf.parms.virt_entry;
     info->load = kernel_elf_load;
+    info->check_overlap = NULL;
 
     if ( elf_check_broken(&info->elf.elf) )
         printk("Xen: warning: ELF kernel broken: %s\n",
diff --git a/xen/arch/arm/kernel.h b/xen/arch/arm/kernel.h
index 1776a4d94d..c900e74df9 100644
--- a/xen/arch/arm/kernel.h
+++ b/xen/arch/arm/kernel.h
@@ -38,6 +38,8 @@ struct kernel_info {
     };
 
     void (*load)(struct kernel_info *info);
+    /* Callback to check overlap between the kernel and the device tree */
+    void (*check_overlap)(struct kernel_info *kinfo);
     int load_attr;
 };