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* [PATCH] xen-mapcache: Avoid entry->lock overflow
@ 2022-01-21 17:33 ` Ross Lagerwall via
  0 siblings, 0 replies; 4+ messages in thread
From: Ross Lagerwall @ 2022-01-21 17:33 UTC (permalink / raw)
  To: Stefano Stabellini, Anthony Perard, Paul Durrant
  Cc: Ross Lagerwall, Igor Druzhinin, Paolo Bonzini, Richard Henderson,
	Eduardo Habkost, Michael S. Tsirkin, Marcel Apfelbaum, xen-devel,
	qemu-devel

In some cases, a particular mapcache entry may be mapped 256 times
causing the lock field to wrap to 0. For example, this may happen when
using emulated NVME and the guest submits a large scatter-gather write.
At this point, the entry map be remapped causing QEMU to write the wrong
data or crash (since remap is not atomic).

Avoid this overflow by increasing the lock field to a uint16_t and also
detect it and abort rather than continuing regardless.

Signed-off-by: Ross Lagerwall <ross.lagerwall@citrix.com>
---
 hw/i386/xen/xen-mapcache.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/hw/i386/xen/xen-mapcache.c b/hw/i386/xen/xen-mapcache.c
index bd47c3d672..82dc495a60 100644
--- a/hw/i386/xen/xen-mapcache.c
+++ b/hw/i386/xen/xen-mapcache.c
@@ -52,7 +52,7 @@ typedef struct MapCacheEntry {
     hwaddr paddr_index;
     uint8_t *vaddr_base;
     unsigned long *valid_mapping;
-    uint8_t lock;
+    uint16_t lock;
 #define XEN_MAPCACHE_ENTRY_DUMMY (1 << 0)
     uint8_t flags;
     hwaddr size;
@@ -355,6 +355,12 @@ tryagain:
     if (lock) {
         MapCacheRev *reventry = g_malloc0(sizeof(MapCacheRev));
         entry->lock++;
+        if (entry->lock == 0) {
+            fprintf(stderr,
+                    "mapcache entry lock overflow: "TARGET_FMT_plx" -> %p\n",
+                    entry->paddr_index, entry->vaddr_base);
+            abort();
+        }
         reventry->dma = dma;
         reventry->vaddr_req = mapcache->last_entry->vaddr_base + address_offset;
         reventry->paddr_index = mapcache->last_entry->paddr_index;
-- 
2.27.0



^ permalink raw reply related	[flat|nested] 4+ messages in thread
* [PATCH] xen-mapcache: Avoid entry->lock overflow
@ 2022-01-21 17:33 ` Ross Lagerwall via
  0 siblings, 0 replies; 4+ messages in thread
From: Ross Lagerwall via @ 2022-01-21 17:33 UTC (permalink / raw)
  To: Stefano Stabellini, Anthony Perard, Paul Durrant
  Cc: Ross Lagerwall, Igor Druzhinin, Paolo Bonzini, Richard Henderson,
	Eduardo Habkost, Michael S. Tsirkin, Marcel Apfelbaum, xen-devel,
	qemu-devel

In some cases, a particular mapcache entry may be mapped 256 times
causing the lock field to wrap to 0. For example, this may happen when
using emulated NVME and the guest submits a large scatter-gather write.
At this point, the entry map be remapped causing QEMU to write the wrong
data or crash (since remap is not atomic).

Avoid this overflow by increasing the lock field to a uint16_t and also
detect it and abort rather than continuing regardless.

Signed-off-by: Ross Lagerwall <ross.lagerwall@citrix.com>
---
 hw/i386/xen/xen-mapcache.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/hw/i386/xen/xen-mapcache.c b/hw/i386/xen/xen-mapcache.c
index bd47c3d672..82dc495a60 100644
--- a/hw/i386/xen/xen-mapcache.c
+++ b/hw/i386/xen/xen-mapcache.c
@@ -52,7 +52,7 @@ typedef struct MapCacheEntry {
     hwaddr paddr_index;
     uint8_t *vaddr_base;
     unsigned long *valid_mapping;
-    uint8_t lock;
+    uint16_t lock;
 #define XEN_MAPCACHE_ENTRY_DUMMY (1 << 0)
     uint8_t flags;
     hwaddr size;
@@ -355,6 +355,12 @@ tryagain:
     if (lock) {
         MapCacheRev *reventry = g_malloc0(sizeof(MapCacheRev));
         entry->lock++;
+        if (entry->lock == 0) {
+            fprintf(stderr,
+                    "mapcache entry lock overflow: "TARGET_FMT_plx" -> %p\n",
+                    entry->paddr_index, entry->vaddr_base);
+            abort();
+        }
         reventry->dma = dma;
         reventry->vaddr_req = mapcache->last_entry->vaddr_base + address_offset;
         reventry->paddr_index = mapcache->last_entry->paddr_index;
-- 
2.27.0



^ permalink raw reply related	[flat|nested] 4+ messages in thread
* Re: [PATCH] xen-mapcache: Avoid entry->lock overflow
  2022-01-21 17:33 ` Ross Lagerwall via
@ 2022-01-21 21:14   ` Stefano Stabellini
  -1 siblings, 0 replies; 4+ messages in thread
From: Stefano Stabellini @ 2022-01-21 21:14 UTC (permalink / raw)
  To: Ross Lagerwall
  Cc: Stefano Stabellini, Anthony Perard, Paul Durrant, Igor Druzhinin,
	Paolo Bonzini, Richard Henderson, Eduardo Habkost,
	Michael S. Tsirkin, Marcel Apfelbaum, xen-devel, qemu-devel

On Fri, 21 Jan 2022, Ross Lagerwall wrote:
> In some cases, a particular mapcache entry may be mapped 256 times
> causing the lock field to wrap to 0. For example, this may happen when
> using emulated NVME and the guest submits a large scatter-gather write.
> At this point, the entry map be remapped causing QEMU to write the wrong
> data or crash (since remap is not atomic).
> 
> Avoid this overflow by increasing the lock field to a uint16_t and also
> detect it and abort rather than continuing regardless.

Nice catch!


> Signed-off-by: Ross Lagerwall <ross.lagerwall@citrix.com>
> ---
>  hw/i386/xen/xen-mapcache.c | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/hw/i386/xen/xen-mapcache.c b/hw/i386/xen/xen-mapcache.c
> index bd47c3d672..82dc495a60 100644
> --- a/hw/i386/xen/xen-mapcache.c
> +++ b/hw/i386/xen/xen-mapcache.c
> @@ -52,7 +52,7 @@ typedef struct MapCacheEntry {
>      hwaddr paddr_index;
>      uint8_t *vaddr_base;
>      unsigned long *valid_mapping;
> -    uint8_t lock;
> +    uint16_t lock;

As struct MapCacheEntry is not packed, we might as well switch to
uint32_t as it doesn't make a difference.


>  #define XEN_MAPCACHE_ENTRY_DUMMY (1 << 0)
>      uint8_t flags;
>      hwaddr size;
> @@ -355,6 +355,12 @@ tryagain:
>      if (lock) {
>          MapCacheRev *reventry = g_malloc0(sizeof(MapCacheRev));
>          entry->lock++;
> +        if (entry->lock == 0) {
> +            fprintf(stderr,
> +                    "mapcache entry lock overflow: "TARGET_FMT_plx" -> %p\n",
> +                    entry->paddr_index, entry->vaddr_base);
> +            abort();

I was going to suggest that it might be better to return NULL and let
the caller handle the failure but it looks like the caller *cannot*
handle the failure.


> +        }
>          reventry->dma = dma;
>          reventry->vaddr_req = mapcache->last_entry->vaddr_base + address_offset;
>          reventry->paddr_index = mapcache->last_entry->paddr_index;
> -- 
> 2.27.0
> 


^ permalink raw reply	[flat|nested] 4+ messages in thread
* Re: [PATCH] xen-mapcache: Avoid entry->lock overflow
@ 2022-01-21 21:14   ` Stefano Stabellini
  0 siblings, 0 replies; 4+ messages in thread
From: Stefano Stabellini @ 2022-01-21 21:14 UTC (permalink / raw)
  To: Ross Lagerwall
  Cc: Eduardo Habkost, Igor Druzhinin, Stefano Stabellini,
	Paul Durrant, Michael S. Tsirkin, Richard Henderson, qemu-devel,
	xen-devel, Anthony Perard, Paolo Bonzini

On Fri, 21 Jan 2022, Ross Lagerwall wrote:
> In some cases, a particular mapcache entry may be mapped 256 times
> causing the lock field to wrap to 0. For example, this may happen when
> using emulated NVME and the guest submits a large scatter-gather write.
> At this point, the entry map be remapped causing QEMU to write the wrong
> data or crash (since remap is not atomic).
> 
> Avoid this overflow by increasing the lock field to a uint16_t and also
> detect it and abort rather than continuing regardless.

Nice catch!


> Signed-off-by: Ross Lagerwall <ross.lagerwall@citrix.com>
> ---
>  hw/i386/xen/xen-mapcache.c | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/hw/i386/xen/xen-mapcache.c b/hw/i386/xen/xen-mapcache.c
> index bd47c3d672..82dc495a60 100644
> --- a/hw/i386/xen/xen-mapcache.c
> +++ b/hw/i386/xen/xen-mapcache.c
> @@ -52,7 +52,7 @@ typedef struct MapCacheEntry {
>      hwaddr paddr_index;
>      uint8_t *vaddr_base;
>      unsigned long *valid_mapping;
> -    uint8_t lock;
> +    uint16_t lock;

As struct MapCacheEntry is not packed, we might as well switch to
uint32_t as it doesn't make a difference.


>  #define XEN_MAPCACHE_ENTRY_DUMMY (1 << 0)
>      uint8_t flags;
>      hwaddr size;
> @@ -355,6 +355,12 @@ tryagain:
>      if (lock) {
>          MapCacheRev *reventry = g_malloc0(sizeof(MapCacheRev));
>          entry->lock++;
> +        if (entry->lock == 0) {
> +            fprintf(stderr,
> +                    "mapcache entry lock overflow: "TARGET_FMT_plx" -> %p\n",
> +                    entry->paddr_index, entry->vaddr_base);
> +            abort();

I was going to suggest that it might be better to return NULL and let
the caller handle the failure but it looks like the caller *cannot*
handle the failure.


> +        }
>          reventry->dma = dma;
>          reventry->vaddr_req = mapcache->last_entry->vaddr_base + address_offset;
>          reventry->paddr_index = mapcache->last_entry->paddr_index;
> -- 
> 2.27.0
> 


^ permalink raw reply	[flat|nested] 4+ messages in thread
end of thread, other threads:[~2022-01-21 21:38 UTC | newest]

Thread overview: 4+ messages (download: mbox.gz / follow: Atom feed)
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2022-01-21 17:33 [PATCH] xen-mapcache: Avoid entry->lock overflow Ross Lagerwall
2022-01-21 17:33 ` Ross Lagerwall via
2022-01-21 21:14 ` Stefano Stabellini
2022-01-21 21:14   ` Stefano Stabellini

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