[Xen-devel] [PATCH] xen/arm64: Correctly compute the virtual address in maddr_to_virt()

[Julien Grall <julien.grall@xxxxxxx>]

The helper maddr_to_virt() is used to translate a machine address to a
virtual address. To save some valuable address space, some part of the
machine address may be compressed.

In theory the PDX code is free to compress any bits so there are no
guarantee the machine index computed will be always greater than
xenheap_mfn_start. This would result to return a virtual address that is
not part of the direct map and trigger a crash at least on debug-build later
on because of the check in virt_to_page().

A recently reverted patch (see 1191156361 "xen/arm: fix mask calculation
in pdx_init_mask") allows the PDX to compress more bits and triggered a
crash on AMD Seattle Platform.

Avoid the crash by keeping track of the base PDX for the xenheap and use
it for computing the virtual address.

Note that virt_to_maddr() does not need to have similar modification as
it is using the hardware to translate the virtual address to a machine
address.

Take the opportunity to fix the ASSERT() as the direct map base address
correspond to the start of the RAM (this is not always 0).

Signed-off-by: Julien Grall <julien.grall@xxxxxxx>

---

With that, the patch 1191156361 "xen/arm: fix mask calculation in
pdx_init_mask" could be re-instated.
---
 xen/arch/arm/mm.c        | 2 ++
 xen/include/asm-arm/mm.h | 6 ++++--
 2 files changed, 6 insertions(+), 2 deletions(-)

diff --git a/xen/arch/arm/mm.c b/xen/arch/arm/mm.c
index 44258ad89c..e1cdeaaf2f 100644
--- a/xen/arch/arm/mm.c
+++ b/xen/arch/arm/mm.c
@@ -165,6 +165,7 @@ mfn_t xenheap_mfn_end __read_mostly;
 vaddr_t xenheap_virt_end __read_mostly;
 #ifdef CONFIG_ARM_64
 vaddr_t xenheap_virt_start __read_mostly;
+unsigned long xenheap_base_pdx __read_mostly;
 #endif
 
 unsigned long frametable_base_pdx __read_mostly;
@@ -796,6 +797,7 @@ void __init setup_xenheap_mappings(unsigned long base_mfn,
     if ( mfn_eq(xenheap_mfn_start, INVALID_MFN) )
     {
         xenheap_mfn_start = _mfn(base_mfn);
+        xenheap_base_pdx = mfn_to_pdx(_mfn(base_mfn));
         xenheap_virt_start = DIRECTMAP_VIRT_START +
             (base_mfn - mfn) * PAGE_SIZE;
     }
diff --git a/xen/include/asm-arm/mm.h b/xen/include/asm-arm/mm.h
index 3dbc8a6469..d6b5544015 100644
--- a/xen/include/asm-arm/mm.h
+++ b/xen/include/asm-arm/mm.h
@@ -135,6 +135,7 @@ extern mfn_t xenheap_mfn_start, xenheap_mfn_end;
 extern vaddr_t xenheap_virt_end;
 #ifdef CONFIG_ARM_64
 extern vaddr_t xenheap_virt_start;
+extern unsigned long xenheap_base_pdx;
 #endif
 
 #ifdef CONFIG_ARM_32
@@ -253,9 +254,10 @@ static inline void *maddr_to_virt(paddr_t ma)
 #else
 static inline void *maddr_to_virt(paddr_t ma)
 {
-    ASSERT(mfn_to_pdx(maddr_to_mfn(ma)) < (DIRECTMAP_SIZE >> PAGE_SHIFT));
+    ASSERT((mfn_to_pdx(maddr_to_mfn(ma)) - xenheap_base_pdx) <
+           (DIRECTMAP_SIZE >> PAGE_SHIFT));
     return (void *)(XENHEAP_VIRT_START -
-                    mfn_to_maddr(xenheap_mfn_start) +
+                    (xenheap_base_pdx << PAGE_SHIFT) +
                     ((ma & ma_va_bottom_mask) |
                      ((ma & ma_top_mask) >> pfn_pdx_hole_shift)));
 }
-- 
2.11.0


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