Re: [Xen-devel] [PATCH v3] xen/grant-table: correctly initialize grant table version 1

[Matt Wilson <msw@xxxxxxxxxx>]

On Tue, Jan 15, 2013 at 01:21:27PM +0000, Matt Wilson wrote:
> Commit 85ff6acb075a484780b3d763fdf41596d8fc0970 (xen/granttable: Grant
> tables V2 implementation) changed the GREFS_PER_GRANT_FRAME macro from
> a constant to a conditional expression. The expression depends on
> grant_table_version being appropriately set. Unfortunately, at init
> time grant_table_version will be 0. The GREFS_PER_GRANT_FRAME
> conditional expression checks for "grant_table_version == 1", and
> therefore returns the number of grant references per frame for v2.
> 
> This causes gnttab_init() to allocate fewer pages for gnttab_list, as
> a frame can old half the number of v2 entries than v1 entries. After
> gnttab_resume() is called, grant_table_version is appropriately
> set. nr_init_grefs will then be miscalculated and gnttab_free_count
> will hold a value larger than the actual number of free gref entries.
> 
> If a guest is heavily utilizing improperly initialized v1 grant
> tables, memory corruption can occur. One common manifestation is
> corruption of the vmalloc list, resulting in a poisoned pointer
> derefrence when accessing /proc/meminfo or /proc/vmallocinfo:
> 
> [   40.770064] BUG: unable to handle kernel paging request at 0000200200001407
> [   40.770083] IP: [<ffffffff811a6fb0>] get_vmalloc_info+0x70/0x110
> [   40.770102] PGD 0
> [   40.770107] Oops: 0000 [#1] SMP
> [   40.770114] CPU 10
> 
> This patch introduces a static variable, grefs_per_grant_frame, to
> cache the calculated value. gnttab_init() now calls
> gnttab_request_version() early so that grant_table_version and
> grefs_per_grant_frame can be appropriately set. A few BUG_ON()s have
> been added to prevent this type of bug from reoccurring in the future.
> 
> Signed-off-by: Matt Wilson <msw@xxxxxxxxxx>
> Reviewed-and-Tested-by: Steven Noonan <snoonan@xxxxxxxxxx>
> Signed-off-by: Ian Campbell <Ian.Campbell@xxxxxxxxxx>

Correction, that should have been "Acked-by: Ian Campbell
<Ian.Campbell@xxxxxxxxxx>". My apologies.

Matt

> Cc: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
> Cc: Annie Li <annie.li@xxxxxxxxxx>
> Cc: xen-devel@xxxxxxxxxxxxx
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> Cc: stable@xxxxxxxxxxxxxxx # v3.3 and newer

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