• Meng Xu's avatar
    libnvdimm, nfit: move the check on nd_reserved2 to the endpoint · 9edcad53
    Meng Xu authored
    Delay the check of nd_reserved2 to the actual endpoint (acpi_nfit_ctl)
    that uses it, as a prevention of a potential double-fetch bug.
    
    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.
    
    Move the validation of the nd_reserved2 field to the ->ndctl()
    implementation where it has a stable buffer to evaluate.
    Signed-off-by: default avatarMeng Xu <mengxu.gatech@gmail.com>
    Signed-off-by: default avatarDan Williams <dan.j.williams@intel.com>
    9edcad53
bus.c 28.2 KB