Messages in this thread | | | Subject | Re: [PATCH v4 0/4] Application Data Integrity feature introduced by SPARC M7 | From | Khalid Aziz <> | Date | Fri, 13 Jan 2017 07:48:42 -0700 |
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On 01/12/2017 06:31 PM, Rob Gardner wrote: > On 01/12/2017 05:22 PM, Khalid Aziz wrote: >> On 01/12/2017 10:53 AM, Dave Hansen wrote: >>> On 01/12/2017 08:50 AM, Khalid Aziz wrote: >>>> 2. Any shared page that has ADI protection enabled on it, must stay ADI >>>> protected across all processes sharing it. >>> >>> Is that true? >>> >>> What happens if a page with ADI tags set is accessed via a PTE without >>> the ADI enablement bit set? >> >> ADI protection applies across all processes in terms of all of them >> must use the same tag to access the shared memory, but if a process >> accesses a shared page with TTE.mcde bit cleared, access will be granted. >> >>> >>>> COW creates an intersection of the two. It creates a new copy of the >>>> shared data. It is a new data page and hence the process creating it >>>> must be the one responsible for enabling ADI protection on it. >>> >>> Do you mean that the application must be responsible? Or the kernel >>> running in the context of the new process must be responsible? >>> >>>> It is also a copy of what was ADI protected data, so should it >>>> inherit the protection instead? >>> >>> I think the COW'd copy must inherit the VMA bit, the PTE bits, and the >>> tags on the cachelines. >>> >>>> I misspoke earlier. I had misinterpreted the results of test I ran. >>>> Changing the tag on shared memory is allowed by memory controller. The >>>> requirement is every one sharing the page must switch to the new tag or >>>> else they get SIGSEGV. >>> >>> I asked this in the last mail, but I guess I'll ask it again. Please >>> answer this directly. >>> >>> If we require that everyone coordinate their tags on the backing >>> physical memory, and we allow a lower-privileged program to access the >>> same data as a more-privileged one, then the lower-privilege app can >>> cause arbitrary crashes in the privileged application. >>> >>> For instance, say sudo mmap()'s /etc/passwd and uses ADI tags to protect >>> the mapping. Couldn't any other app in the system prevent sudo from >>> working? >>> >>> How can we *EVER* allow tags to be set on non-writable mappings? > > I don't think you can write a tag to memory if you don't have write > access in the TTE. Writing a tag requires a store instruction, and if > the machine is at all sane, this will fault if you don't have write access. >
But could you have mmap'd the file writable, set the tags and then changed the protection on memory to read-only? That would be the logical way to ADI protect a memory being used to mmap a file. Right?
-- Khalid
> Rob > > > >> >> I understand your quetion better now. That is a very valid concern. >> Using ADI tags to prevent an unauthorized process from just reading >> data in memory, say an in-memory copy of database, is one of the use >> cases for ADI. This means there is a reasonable case to allow enabling >> ADI and setting tags even on non-writable mappings. On the other hand, >> if an unauthorized process manages to map the right memory pages in >> its address space, it can read them any way by not setting TTE.mcd. >> >> Userspace app can set tag on any memory it has mapped in without >> requiring assistance from kernel. Can this problem be solved by not >> allowing setting TTE.mcd on non-writable mappings? Doesn't the same >> problem occur on writable mappings? If a privileged process mmap()'s a >> writable file with MAP_SHARED, enables ADI and sets tag on the mmap'd >> memory region, then another lower privilege process mmap's the same >> file writable (assuming file permissions allow it to), enables ADI and >> sets a different tag on it, the privileged process would get SIGSEGV >> when it tries to access the mmap'd file. Right? > > > > -- > To unsubscribe from this list: send the line "unsubscribe sparclinux" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html
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