On Wed, Sep 04, 2024 at 09:50:56AM +0200, Christophe Leroy wrote:

Hi,

Le 02/09/2024 à 07:31, Omar Sandoval a écrit :

quoted

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From: Omar Sandoval <redacted>

I found that on x86, copy_to_kernel_nofault() still faults on addresses
outside of the kernel address range (including NULL):

   # echo ttyS0 > /sys/module/kgdboc/parameters/kgdboc
   # echo g > /proc/sysrq-trigger
   ...
   [15]kdb> mm 0 1234
   [   94.652476] BUG: kernel NULL pointer dereference, address: 0000000000000000

...

quoted

Note that copy_to_kernel_nofault() uses pagefault_disable(), but it
still faults. This is because with Supervisor Mode Access Prevention
(SMAP) enabled, do_user_addr_fault() Oopses on a fault for a user
address from kernel space _before_ checking faulthandler_disabled().

copy_from_kernel_nofault() avoids this by checking that the address is
in the kernel before doing the actual memory access. Do the same in
copy_to_kernel_nofault() so that we get an error as expected:

   # echo ttyS0 > /sys/module/kgdboc/parameters/kgdboc
   # echo g > /proc/sysrq-trigger
   ...
   [17]kdb> mm 0 1234
   kdb_putarea_size: Bad address 0x0
   diag: -21: Invalid address

Signed-off-by: Omar Sandoval <redacted>
---
  mm/maccess.c | 3 +++
  1 file changed, 3 insertions(+)

diff --git a/mm/maccess.c b/mm/maccess.c
index 72e9c03ea37f..d67dee51a1cc 100644
--- a/mm/maccess.c
+++ b/mm/maccess.c

@@ -61,6 +61,9 @@ long copy_to_kernel_nofault(void *dst, const void *src, size_t size)
         if (!IS_ENABLED(CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS))
                 align = (unsigned long)dst | (unsigned long)src;

+       if (!copy_kernel_nofault_allowed(dst, size))
+               return -ERANGE;
+
         pagefault_disable();
         if (!(align & 7))
                 copy_to_kernel_nofault_loop(dst, src, size, u64, Efault);
--

2.46.0

This patch leads to the following errors on ppc64le_defconfig:

[    2.423930][    T1] Running code patching self-tests ...
[    2.428912][    T1] code-patching: test failed at line 395
[    2.429085][    T1] code-patching: test failed at line 398
[    2.429561][    T1] code-patching: test failed at line 432
[    2.429679][    T1] code-patching: test failed at line 435

This seems to be linked to commit c28c15b6d28a ("powerpc/code-patching: Use
temporary mm for Radix MMU"), copy_from_kernel_nofault_allowed() returns
false for the patching area.

Thanks for testing. This patch isn't worth the trouble, so we can drop
it.

Thanks,
Omar