On 22.09.2021 11:26, Roger Pau Monné wrote: > On Tue, Sep 21, 2021 at 12:12:05PM +0200, Jan Beulich wrote: >> On 21.09.2021 10:32, Roger Pau Monné wrote: >>> On Mon, Sep 20, 2021 at 05:27:17PM +0200, Jan Beulich wrote: >>>> On 20.09.2021 12:20, Roger Pau Monné wrote: >>>>> On Mon, Sep 13, 2021 at 08:41:47AM +0200, Jan Beulich wrote: >>>>>> --- a/xen/include/asm-arm/grant_table.h >>>>>> +++ b/xen/include/asm-arm/grant_table.h >>>>>> + if ( gfn_eq(ogfn, INVALID_GFN) || gfn_eq(ogfn, gfn) || >>>>>> \ >>>>> >>>>> I'm slightly confused by this checks, don't you need to check for >>>>> gfn_eq(gfn, INVALID_GFN) (not ogfn) in order to call >>>>> guest_physmap_remove_page? >>>> >>>> Why? It's ogfn which gets passed to the function. And it indeed is the >>>> prior GFN's mapping that we want to remove here. >>>> >>>>> Or assuming that ogfn is not invalid can be used to imply a removal? >>>> >>>> That implication can be (and on x86 is) used for the incoming argument, >>>> i.e. "gfn". I don't think "ogfn" can serve this purpose. >>> >>> I guess I'm confused due to the ogfn checks done on the Arm side that >>> are not performed on x86. So on Arm you always need to explicitly >>> unhook the previous GFN before attempting to setup a new mapping, >>> while on x86 you only need to do this when it's a removal in order to >>> clear the entry? >> >> The difference isn't with guest_physmap_add_entry() (both x86 and >> Arm only insert a new mapping there), but with >> xenmem_add_to_physmap_one(): Arm's variant doesn't care about prior >> mappings. And gnttab_map_frame() gets called only from there. This >> is effectively what the first paragraph of the description is about. > > OK, sorry, it wasn't clear to me from the description. Could you > explicitly mention in the description that the removal is moved into > gnttab_set_frame_gfn on Arm in order to cope with the fact that > xenmem_add_to_physmap_one doesn't perform it.
Well, it's not really "in order to cope" - that's true for the placement prior to this change as well, so not a justification for the change. Nevertheless I've tried to make this more clear by changing the 1st paragraph to: "Without holding appropriate locks, attempting to remove a prior mapping of the underlying page is pointless, as the same (or another) mapping could be re-established by a parallel request on another vCPU. Move the code to Arm's gnttab_set_frame_gfn(); it cannot be dropped there since xenmem_add_to_physmap_one() doesn't call it either (unlike on x86). Of course this new placement doesn't improve things in any way as far as the security of grant status frame mappings goes (see XSA-379). Proper locking would be needed here to allow status frames to be mapped securely." > TBH I think it would be in our best interest to try to make > xenmem_add_to_physmap_one behave as close as possible between arches. > This discrepancy between x86 and Arm regarding page removal is just > going to bring more trouble in the long term, and hiding the > differences inside gnttab_set_frame_gfn just makes it even more > obscure. Stefano, Julien? Jan
