[Xen-devel] [PATCH v2 1/2] xen/arm: Rework the way to compute dom0 DTB base address

[Julien Grall <julien.grall@xxxxxxxxxx>]

If the DTB is loading right the 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@xxxxxxxxxx>

Changes in v2:
    - Align the DTB base address to 2Mib
    - Add dtb_check_overlap to check if the kernel will overlap the DTB
    - Fix typo
---
 xen/arch/arm/domain_build.c |   51 ++++++++++++++++++++++++++++++++++++++-----
 xen/arch/arm/kernel.c       |    2 ++
 xen/arch/arm/kernel.h       |    7 ++++++
 3 files changed, 54 insertions(+), 6 deletions(-)

diff --git a/xen/arch/arm/domain_build.c b/xen/arch/arm/domain_build.c
index b92c64b..515fabe 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 +503,25 @@ 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 to linux (Linux use
+     * the space after it to decompress)
+     * Load the DTB at the end of the first bank or below 4Gib
      */
-    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);
+    /* Linux requires the address to be aligned to 2Mb */
+    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:
@@ -521,9 +530,36 @@ static int prepare_dtb(struct domain *d, struct 
kernel_info *kinfo)
     return -EINVAL;
 }
 
+static int dtb_check_overlap(struct kernel_info *kinfo)
+{
+    paddr_t zimage_start = kinfo->zimage.load_addr;
+    paddr_t zimage_end = kinfo->zimage.load_addr + kinfo->zimage.len;
+    paddr_t dtb_start = kinfo->dtb_paddr;
+    paddr_t dtb_end = kinfo->dtb_paddr + fdt_totalsize(kinfo->fdt);
+
+    /*
+     * Check the kernel won't overlap the kernel
+     * Only when it's a ZIMAGE
+     */
+    if ( kinfo->type != KERNEL_ZIMAGE )
+        return 0;
+
+    if ( !(MAX(zimage_start, dtb_end) < MIN(zimage_end, dtb_start)) )
+        return 0;
+
+    printk(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);
+    xfree(kinfo->fdt);
+
+    return -EINVAL;
+}
+
 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 +595,13 @@ int construct_dom0(struct domain *d)
     if ( rc < 0 )
         return rc;
 
+    rc = dtb_check_overlap(&kinfo);
+    if ( rc < 0 )
+        return rc;
+
     /* 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 8f4a60d..f8c8850 100644
--- a/xen/arch/arm/kernel.c
+++ b/xen/arch/arm/kernel.c
@@ -152,6 +152,7 @@ static int kernel_try_zimage_prepare(struct kernel_info 
*info,
 
     info->entry = info->zimage.load_addr;
     info->load = kernel_zimage_load;
+    info->type = KERNEL_ZIMAGE;
 
     return 0;
 }
@@ -193,6 +194,7 @@ static int kernel_try_elf_prepare(struct kernel_info *info,
      */
     info->entry = info->elf.parms.virt_entry;
     info->load = kernel_elf_load;
+    info->type = KERNEL_ELF;
 
     return 0;
 err:
diff --git a/xen/arch/arm/kernel.h b/xen/arch/arm/kernel.h
index 1776a4d..b4ecd30 100644
--- a/xen/arch/arm/kernel.h
+++ b/xen/arch/arm/kernel.h
@@ -9,6 +9,12 @@
 #include <xen/libelf.h>
 #include <xen/device_tree.h>
 
+enum kernel_type
+{
+    KERNEL_ELF,
+    KERNEL_ZIMAGE,
+};
+
 struct kernel_info {
 #ifdef CONFIG_ARM_64
     enum domain_type type;
@@ -23,6 +29,7 @@ struct kernel_info {
 
     void *kernel_img;
     unsigned kernel_order;
+    enum kernel_type type;
 
     union {
         struct {
-- 
1.7.10.4


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