Messages in this thread | | | Subject | Re: [PATCH -next] uaccess: fix __access_ok limit setup in compat mode | From | "chenjiahao (C)" <> | Date | Tue, 22 Mar 2022 20:55:56 +0800 |
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在 2022/3/18 15:44, Arnd Bergmann 写道: > On Fri, Mar 18, 2022 at 8:11 AM Chen Jiahao <chenjiahao16@huawei.com> wrote: >> In __access_ok, TASK_SIZE_MAX is used to check if a memory access >> is in user address space, but some cases may get omitted in compat >> mode. >> >> For example, a 32-bit testcase calling pread64(fd, buf, -1, 1) >> and running in x86-64 kernel, the obviously illegal size "-1" will >> get ignored by __access_ok. Since from the kernel point of view, >> 32-bit userspace 0xffffffff is within the limit of 64-bit >> TASK_SIZE_MAX. >> >> Replacing the limit TASK_SIZE_MAX with TASK_SIZE in __access_ok >> will fix the problem above. > I don't see what problem this fixes, the choice of TASK_SIZE_MAX in > __access_ok() is intentional, as this means we can use a compile-time > constant as the limit, which produces better code. > > Any user pointer between COMPAT_TASK_SIZE and TASK_SIZE_MAX is > not accessible by a user process but will not let user space access > any kernel data either, which is the point of the check. > > In your example of using '-1' as the pointer, access_ok() returns true, > so the kernel can go on to perform an unchecked __get_user() on > __put_user() on 0xffffffffull, which causes page fault that is intercepted > by the ex_table fixup. > > This should not result in any user visible difference, in both cases > user process will see a -EFAULT return code from its system call. > Are you able to come up with a test case that shows an observable > difference in behavior? > > Arnd > > .
Actually, this patch do comes from a testcase failure, the code is pasted below:
#define TMPFILE "__1234567890" #define BUF_SIZE 1024
int main() { char buf[BUF_SIZE] = {0}; int fd; int ret; int err;
fd = open(TMPFILE, O_CREAT | O_RDWR); if(-1 == fd) { perror("open"); return 1; }
ret = pread64(fd, buf, -1, 1); if((-1 == ret) && (EFAULT == errno)) { close(fd); unlink(TMPFILE); printf("PASS\n"); return 0; } err = errno; perror("pread64"); printf("err = %d\n", err); close(fd); unlink(TMPFILE); printf("FAIL\n");
return 1; }
The expected result is:
PASS
but the result of 32-bit testcase running in x86-64 kernel with compat mode is:
pread64: Success err = 0 FAIL
In my explanation, pread64 is called with count '0xffffffffull' and offset '1', which might still not trigger
page fault in 64-bit kernel.
This patch uses TASK_SIZE as the addr_limit to performance a stricter address check and intercepts
the illegal pointer address from 32-bit userspace at a very early time. Which is roughly the same
address limit check as __access_ok in arch/ia64.
This is why this fixes my testcase failure above, or have I missed anything else?
Jiahao
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