powerpc: Punch a hole in /dev/mem for librtas

With CONFIG_STRICT_DEVMEM=y, user space cannot read any part of /dev/mem.
Since this breaks librtas, punch a hole in /dev/mem to allow access to the
rmo_buffer that librtas needs.

Anton Blanchard reported the problem and helped with the fix.

A quick test for this patch:

       # cat /proc/rtas/rmo_buffer
       000000000f190000 10000

       # python -c "print 0x000000000f190000 / 0x10000"
       3865

       # dd if=/dev/mem of=/tmp/foo count=1 bs=64k skip=3865
       1+0 records in
       1+0 records out
       65536 bytes (66 kB) copied, 0.000205235 s, 319 MB/s

       # dd if=/dev/mem of=/tmp/foo
       dd: reading `/dev/mem': Operation not permitted
       0+0 records in
       0+0 records out
       0 bytes (0 B) copied, 0.00022519 s, 0.0 kB/s

Signed-off-by: Sukadev Bhattiprolu <sukadev@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

2 files changed, 15 insertions, 0 deletions

diff --git a/arch/powerpc/include/asm/rtas.h b/arch/powerpc/include/asm/rtas.h
index 1646b76bd3d2..01c143bb77ae 100644
--- a/arch/powerpc/include/asm/rtas.h
+++ b/arch/powerpc/include/asm/rtas.h
@@ -313,5 +313,17 @@ static inline u32 rtas_config_addr(int busno, int devfn, int reg)
 extern void __cpuinit rtas_give_timebase(void);
 extern void __cpuinit rtas_take_timebase(void);
 
+#ifdef CONFIG_PPC_RTAS
+static inline int page_is_rtas_user_buf(unsigned long pfn)
+{
+        unsigned long paddr = (pfn << PAGE_SHIFT);
+        if (paddr >= rtas_rmo_buf && paddr < (rtas_rmo_buf + RTAS_RMOBUF_MAX))
+                return 1;
+        return 0;
+}
+#else
+static inline int page_is_rtas_user_buf(unsigned long pfn) { return 0;}
+#endif
+
 #endif /* __KERNEL__ */
 #endif /* _POWERPC_RTAS_H */
diff --git a/arch/powerpc/mm/mem.c b/arch/powerpc/mm/mem.c
index 83d819f30864..395f42d5f785 100644
--- a/arch/powerpc/mm/mem.c
+++ b/arch/powerpc/mm/mem.c
@@ -51,6 +51,7 @@
 #include <asm/vdso.h>
 #include <asm/fixmap.h>
 #include <asm/swiotlb.h>
+#include <asm/rtas.h>
 
 #include "mmu_decl.h"
 
@@ -600,6 +601,8 @@ int devmem_is_allowed(unsigned long pfn)
                 return 0;
         if (!page_is_ram(pfn))
                 return 1;
+        if (page_is_rtas_user_buf(pfn))
+                return 1;
         return 0;
 }
 #endif /* CONFIG_STRICT_DEVMEM */