Messages in this thread | | | Date | Thu, 16 Nov 2023 09:58:59 -0500 | From | Mark Rutland <> | Subject | Re: [PATCH] arm64: Fix 32-bit compatible userspace write size overflow error |
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On Thu, Nov 16, 2023 at 03:47:05PM +0800, Jinjie Ruan wrote: > For 32-bit compatible userspace program, write with size = -1 return not > -1 but unexpected other values, which is due to the __access_ok() check is > not right.
Can you please explain why you believe that is unexpected?
e.g. Is that documented somewhere? Do you see a real application depending on that somewhow?
> The specified "addr + size" is greater than 32-bit limit and > should return -EFAULT, but TASK_SIZE_MAX still defined as UL(1) << VA_BITS > in U32 mode, which is much greater than "addr + size" and cannot catch the > overflow error.
The check against TASK_SIZE_MAX is not intended to catch 32-bit addr + size overflow; it's intended to check that uaccesses never touch kernel memory. The kernel's uaccess routines use 64-bit (or 65-bit) arithmetic, so these won't wrap and access memory at the start of the user address space.
> Fix above error by checking 32-bit limit if it is 32-bit compatible > userspace program. > > How to reproduce: > > The test program is as below: > > cat test.c > #include <unistd.h> > #include <fcntl.h> > #include <stdio.h> > #include <stdint.h> > #include <stdlib.h> > #include <assert.h> > > #define pinfo(fmt, args...) \ > fprintf(stderr, "[INFO][%s][%d][%s]:"fmt, \ > __FILE__,__LINE__,__func__,##args) > > #undef SIZE_MAX > #define SIZE_MAX -1 > > int main() > { > char wbuf[3] = { 'x', 'y', 'z' }; > char *path = "write.tmp"; > int ret; > > int fd = open(path, O_RDWR | O_CREAT); > if (fd<0) > { > pinfo("fd=%d\n", fd); > exit(-1); > } > > assert(write(fd, wbuf, 3) == 3); > > ret = write (fd, wbuf, SIZE_MAX); > pinfo("ret=%d\n", ret); > pinfo("size_max=%d\n",SIZE_MAX); > assert(ret==-1); > close(fd); > pinfo("INFO: end\n"); > > return 0; > } > > aarch64-linux-gnu-gcc --static test.c -o test > arm-linux-gnueabi-gcc --static test.c -o test32 > > Before applying this patch, userspace 32-bit program return 1112 if the > write size = -1 as below: > /root # ./test > [INFO][test.c][32][main]:ret=-1 > [INFO][test.c][33][main]:size_max=-1 > [INFO][test.c][36][main]:INFO: end > /root # ./test32 > [INFO][test.c][32][main]:ret=1112 > [INFO][test.c][33][main]:size_max=-1 > test32: test.c:34: main: Assertion `ret==-1' failed. > Aborted > > After applying this patch, userspace 32-bit program return -1 if the write > size = -1 as expected as below: > /root # ./test > [INFO][test.c][32][main]:ret=-1 > [INFO][test.c][33][main]:size_max=-1 > [INFO][test.c][36][main]:INFO: end > /root # ./test32 > [INFO][test.c][32][main]:ret=-1 > [INFO][test.c][33][main]:size_max=-1 > [INFO][test.c][36][main]:INFO: end > > Fixes: 967747bbc084 ("uaccess: remove CONFIG_SET_FS")
As above, this is *not* a fix. This is the intended behaviour.
AFAICT, the behaviour didn't change on arm64 in that commit either; we were unconditionally using TASK_SIZE_MAX many commits earlier, e.g. in commit:
3d2403fd10a1dbb3 ("arm64: uaccess: remove set_fs()")
... so the fixes tag is bogus on both fronts.
> Signed-off-by: Jinjie Ruan <ruanjinjie@huawei.com> > --- > arch/arm64/include/asm/processor.h | 5 +++++ > 1 file changed, 5 insertions(+) > > diff --git a/arch/arm64/include/asm/processor.h b/arch/arm64/include/asm/processor.h > index e5bc54522e71..6a087d58a90a 100644 > --- a/arch/arm64/include/asm/processor.h > +++ b/arch/arm64/include/asm/processor.h > @@ -52,7 +52,12 @@ > > #define DEFAULT_MAP_WINDOW_64 (UL(1) << VA_BITS_MIN) > #define TASK_SIZE_64 (UL(1) << vabits_actual) > +#ifdef CONFIG_COMPAT > +#define TASK_SIZE_MAX (test_thread_flag(TIF_32BIT) ? \ > + UL(0x100000000) : (UL(1) << VA_BITS)) > +#else > #define TASK_SIZE_MAX (UL(1) << VA_BITS) > +#endif
This isn't even the same as on 32-bit. On 32-bit arm, the task size split can be 1G/3G, 2G/2G, or 3G/1G depending on configuration, and 4G/4G isn't currently an option.
I don't believe that userspace is actually dependent upon this for functional reasons, and I don't believe that there's a security issue here. Even if access_ok() allows addr+size to go past 4G, the kernel address calculations are 64-bit and won't wrap.
For all the reasons above, I don't beleive this is correct nor do I believe this is necesssary. Given that, NAK to this patch.
Thanks, Mark.
> > #ifdef CONFIG_COMPAT > #if defined(CONFIG_ARM64_64K_PAGES) && defined(CONFIG_KUSER_HELPERS) > -- > 2.34.1 >
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