Messages in this thread | | | Subject | Re: [PATCH] nvdimm: fix potential double-fetch bug | From | Meng Xu <> | Date | Tue, 12 Sep 2017 18:49:54 -0400 |
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Hi Jerry,
Thank you for the question. Yes, these double copies do seem to present an issue.
__nd_ioctl() and acpi_nfit_ctl() both use the same way to derive `out_size`, but based on different data fetches.
A simple patch would be memcmp(buf, in_env, in_len) memcmp(buf + in_len, out_env, out_len)
I am not sure I captured all the subtle issues with such a patch so please allow me some time to create and test it.
Best regards, Meng
On 09/12/2017 06:03 PM, Jerry Hoemann wrote: > On Thu, Aug 31, 2017 at 03:42:52PM -0700, Dan Williams wrote: >> [ adding Jerry ] >> >> On Wed, Aug 23, 2017 at 2:07 PM, Meng Xu <meng.xu@gatech.edu> wrote: >>> From: Meng Xu <mengxu.gatech@gmail.com> >>> >>> While examining the kernel source code, I found a dangerous operation that >>> could turn into a double-fetch situation (a race condition bug) where >>> the same userspace memory region are fetched twice into kernel with sanity >>> checks after the first fetch while missing checks after the second fetch. >>> >>> In the case of _IOC_NR(ioctl_cmd) == ND_CMD_CALL: >>> >>> 1. The first fetch happens in line 935 copy_from_user(&pkg, p, sizeof(pkg) >>> >>> 2. subsequently `pkg.nd_reserved2` is asserted to be all zeroes >>> (line 984 to 986). >>> >>> 3. The second fetch happens in line 1022 copy_from_user(buf, p, buf_len) >>> >>> 4. Given that `p` can be fully controlled in userspace, an attacker can >>> race condition to override the header part of `p`, say, >>> `((struct nd_cmd_pkg *)p)->nd_reserved2` to arbitrary value >>> (say nine 0xFFFFFFFF for `nd_reserved2`) after the first fetch but before the >>> second fetch. The changed value will be copied to `buf`. >>> >>> 5. There is no checks on the second fetches until the use of it in >>> line 1034: nd_cmd_clear_to_send(nvdimm_bus, nvdimm, cmd, buf) and >>> line 1038: nd_desc->ndctl(nd_desc, nvdimm, cmd, buf, buf_len, &cmd_rc) >>> which means that the assumed relation, `p->nd_reserved2` are all zeroes might >>> not hold after the second fetch. And once the control goes to these functions >>> we lose the context to assert the assumed relation. >>> >>> 6. Based on my manual analysis, `p->nd_reserved2` is not used in function >>> `nd_cmd_clear_to_send` and potential implementations of `nd_desc->ndctl` >>> so there is no working exploit against it right now. However, this could >>> easily turns to an exploitable one if careless developers start to use >>> `p->nd_reserved2` later and assume that they are all zeroes. >>> >>> Proposed patch: >>> >>> The patch explicitly overrides `buf->nd_reserved2` after the second fetch with >>> the value `pkg.nd_reserved2` from the first fetch. In this way, it is assured >>> that the relation, `buf->nd_reserved2` are all zeroes, holds after the second >>> fetch. >>> >>> Signed-off-by: Meng Xu <mengxu.gatech@gmail.com> >>> --- >>> drivers/nvdimm/bus.c | 6 ++++++ >>> 1 file changed, 6 insertions(+) >>> >>> >>> >>> diff --git a/drivers/nvdimm/bus.c b/drivers/nvdimm/bus.c >>> index 937fafa..20c4d0f 100644 >>> --- a/drivers/nvdimm/bus.c >>> +++ b/drivers/nvdimm/bus.c >>> @@ -1024,6 +1024,12 @@ static int __nd_ioctl(struct nvdimm_bus *nvdimm_bus, struct nvdimm *nvdimm, >>> goto out; >>> } >>> >>> + if (cmd == ND_CMD_CALL) { >>> + struct nd_cmd_pkg *hdr = (struct nd_cmd_pkg *)buf; >>> + memcpy(hdr->nd_reserved2, pkg.nd_reserved2, >>> + sizeof(pkg.nd_reserved2)); >>> + } >>> + >> I think we're ok because the end point like acpi_nfit_ctl() is >> responsible for re-validating the buffer. So what I would rather like >> to see is deleting this loop: >> >> for (i = 0; i < ARRAY_SIZE(pkg.nd_reserved2); i++) >> if (pkg.nd_reserved2[i]) >> return -EINVAL; >> >> ...from __nd_ioctl() and move it into acpi_nfit_ctl() directly where it belongs. > Sorry for the delay, I've been away. > > I'm okay with moving the test to the beginning of acpi_nfit_ctl. If/When the reserved > fields are defined/used, we may need to tweak that. But we can cross that > bridge when it comes. > > However, I do have a question. > > There are two for loops in __nd_ioctl that process desc->in_num and desc->out_num > respectively. These loops also copy_from_user before > > buf = vmalloc(buf_len); > if (!buf) > return -ENOMEM; > > if (copy_from_user(buf, p, buf_len)) { > rc = -EFAULT; > goto out; > } > > > Do these double copy instances present any problems? > >
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