Arm: correct FIXADDR_TOP

While reviewing a RISC-V patch cloning the Arm code, I noticed an
off-by-1 here: FIX_PMAP_{BEGIN,END} being an inclusive range and
FIX_LAST being the same as FIX_PMAP_END, FIXADDR_TOP cannot derive from
FIX_LAST alone, or else the BUG_ON() in virt_to_fix() would trigger if
FIX_PMAP_END ended up being used.

While touching this area also add a check for fixmap and boot FDT area
to not only not overlap, but to have at least one (unmapped) page in
between.

Fixes: 4f17357b52f6 ("xen/arm: add Persistent Map (PMAP) infrastructure")
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Michal Orzel <michal.orzel@amd.com>
master commit: fe3412ab83cc53c2bf2c497be3794bc09751efa5
master date: 2024-08-13 21:50:55 +0100

diff --git a/xen/arch/arm/include/asm/fixmap.h b/xen/arch/arm/include/asm/fixmap.h
index 734eb9b1d42c68f28ba5933a7ba35dccc0e64236..c92594c438224211bb9dff00710c60e26982c4cf 100644 (file)
--- a/xen/arch/arm/include/asm/fixmap.h
+++ b/xen/arch/arm/include/asm/fixmap.h
@@ -18,7 +18,7 @@
 #define FIXMAP_LAST FIXMAP_PMAP_END
 
 #define FIXADDR_START FIXMAP_ADDR(0)
-#define FIXADDR_TOP FIXMAP_ADDR(FIXMAP_LAST)
+#define FIXADDR_TOP FIXMAP_ADDR(FIXMAP_LAST + 1)
 
 #ifndef __ASSEMBLY__
 

diff --git a/xen/arch/arm/mm.c b/xen/arch/arm/mm.c
index c34cc94c907ad02a2448f15f9105c02151ec22e5..1ff67ff2b5bea1eb76c35cfc4ad36b77216891bc 100644 (file)
--- a/xen/arch/arm/mm.c
+++ b/xen/arch/arm/mm.c
@@ -198,6 +198,12 @@ static void __init __maybe_unused build_assertions(void)
 
 #undef CHECK_SAME_SLOT
 #undef CHECK_DIFFERENT_SLOT
+
+    /*
+     * Fixmaps must not overlap with boot FDT mapping area. Make sure there's
+     * at least one guard page in between.
+     */
+    BUILD_BUG_ON(FIXADDR_TOP >= BOOT_FDT_VIRT_START);
 }
 
 static lpae_t *xen_map_table(mfn_t mfn)