Re: [PATCH] mm: fix potential invalid pointer dereference in kmemdup()

From: Xujun Leng
Date: Thu Mar 09 2023 - 01:47:14 EST

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> On 07.03.23 10:03, Xujun Leng wrote:
> > If kmemdup() was called with src == NULL, then memcpy() source address
> > is fatal, and if kmemdup() was called with len == 0, kmalloc_track_caller()
> > will return ZERO_SIZE_PTR to variable p, then memcpy() destination address
> > is fatal. Both 2 cases will cause an invalid pointer dereference.
> >

> "fix" in subject implies that there is actually a case broken. Is there,
> or is this rather a "sanitize" ?
Yes, I agree that word "sanitize" is a better choice.
And no, I don't find an actually case but in my test code as follow:

#include <linux/module.h>
#include <linux/string.h>
#include <linux/slab.h>
#include <linux/printk.h>
#include <linux/err.h>

/*
* Test cases for kmemdup() and memdup_user().
*/
enum {
TC_KMEMDUP_ARG0_NULL, /* i.e. kmemdup(NULL, 5, GFP_KERNEL) */
TC_KMEMDUP_ARG1_ZERO, /* i.e. kmemdup("12345", 0, GFP_KERNEL) */

TC_MEMDUP_USER_ARG0_NULL, /* i.e. memdup_user(NULL, 5) */
TC_MEMDUP_USER_ARG1_ZERO /* i.e. memdup_user("12345", 0) */
};

static int test_case;
static const char *test_func_name[] = {"kmemdup", "memdup_user"};
static void *ptr;

module_param(test_case, int, S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH);

static void *kmemdup_arg0_null(void)
{
return kmemdup(NULL, 5, GFP_KERNEL);
}

static void *kmemdup_arg1_zero(void)
{
return kmemdup("12345", 0, GFP_KERNEL);
}

static void *memdup_user_arg0_null(void)
{
return memdup_user(NULL, 5);
}

static void *memdup_user_arg1_zero(void)
{
return memdup_user("12345", 0);
}

static int check_ptr(void)
{
if (ZERO_OR_NULL_PTR(ptr)) {
printk(KERN_ERR "test case %d: %s failed, PTR_ERR(ptr) = %ld\n",
test_case, test_func_name[test_case / 2], PTR_ERR(ptr));
return -EINVAL;
}

if (IS_ERR(ptr)) {
printk(KERN_ERR "test case %d: %s failed, PTR_ERR(ptr) = %ld\n",
test_case, test_func_name[test_case / 2], PTR_ERR(ptr));
return PTR_ERR(ptr);
}

printk(KERN_INFO "mm-util test module loaded.\n");

return 0;
}

static int __init memdup_user_test_init(void)
{
if (test_case < 0 || test_case > TC_MEMDUP_USER_ARG1_ZERO) {
printk(KERN_INFO "invalid test case %d\n", test_case);
return -EINVAL;
}

printk(KERN_INFO "test case: %d\n", test_case);

switch (test_case) {
case TC_KMEMDUP_ARG0_NULL:
ptr = kmemdup_arg0_null();
break;
case TC_KMEMDUP_ARG1_ZERO:
ptr = kmemdup_arg1_zero();
break;

case TC_MEMDUP_USER_ARG0_NULL:
ptr = memdup_user_arg0_null();
break;

case TC_MEMDUP_USER_ARG1_ZERO:
ptr = memdup_user_arg1_zero();
break;

default:
/* should be never happend */
ptr = NULL;
break;
}

return check_ptr();
}

static void __exit memdup_user_test_exit(void)
{
if (ptr) {
kfree(ptr);
ptr = NULL;
}

printk(KERN_INFO "mm-util test module exited.\n");
}

module_init(memdup_user_test_init);
module_exit(memdup_user_test_exit);

MODULE_LICENSE("GPL");

Build the code as module, and run the module in QEMU ARM64, with different
test case(pass 0,1,2,3 to moddule parameter "test_case"), get follow the
results:

root@qemu-ubuntu:~# modprobe memdup_kernel_user_test test_case=0
[ 142.979506] Unable to handle kernel NULL pointer dereference at virtual address 0000000000000000
[ 142.983171] Mem abort info:
[ 142.984049] ESR = 0x0000000096000004
[ 142.984556] EC = 0x25: DABT (current EL), IL = 32 bits
[ 142.985327] SET = 0, FnV = 0
[ 142.986867] EA = 0, S1PTW = 0
[ 142.987198] FSC = 0x04: level 0 translation fault
[ 142.987555] Data abort info:
[ 142.987819] ISV = 0, ISS = 0x00000004
[ 142.988132] CM = 0, WnR = 0
[ 142.988540] user pgtable: 4k pages, 48-bit VAs, pgdp=0000000046168000
[ 142.989715] [0000000000000000] pgd=0000000000000000, p4d=0000000000000000
[ 142.992158] Internal error: Oops: 0000000096000004 [#1] PREEMPT SMP
[ 142.993012] Modules linked in: memdup_kernel_user_test(+) drm ip_tables x_tables ipv6
[ 142.996663] CPU: 0 PID: 133 Comm: modprobe Not tainted 6.3.0-rc1-next-20230307-dirty #1
[ 143.002024] Hardware name: linux,dummy-virt (DT)
[ 143.003370] pstate: 80000005 (Nzcv daif -PAN -UAO -TCO -DIT -SSBS BTYPE=--)
[ 143.005461] pc : __memcpy+0x54/0x230
[ 143.006833] lr : kmemdup+0x50/0x68
[ 143.007208] sp : ffff80000aa53ae0
[ 143.011440] x29: ffff80000aa53ae0 x28: ffff8000010c0378 x27: ffff8000010c0058
[ 143.012386] x26: ffff80000a216fd8 x25: ffff80000aa53d00 x24: ffff8000010c0040
[ 143.014183] x23: 0000000000000000 x22: ffff0000037d6580 x21: 0000000000000000
[ 143.018590] x20: 0000000000000005 x19: ffff0000039a9100 x18: 0000000000000001
[ 143.020166] x17: ffff80000aa75000 x16: ffff0000047bed91 x15: ffff0000037d69f8
[ 143.021158] x14: 0000000000000147 x13: ffff0000037d69f8 x12: 00000000ffffffea
[ 143.024978] x11: 00000000ffffefff x10: 00000000ffffefff x9 : ffff80000a1fb518
[ 143.025800] x8 : 00000000ffffffff x7 : 00000000ffffffff x6 : ffff800036288000
[ 143.026667] x5 : ffff0000039a9105 x4 : 0000000000000005 x3 : 0000000080200020
[ 143.027257] x2 : 0000000000000005 x1 : 0000000000000000 x0 : ffff0000039a9100
[ 143.028177] Call trace:
[ 143.028833] __memcpy+0x54/0x230
[ 143.029424] memdup_user_test_init+0xd8/0x1000 [memdup_kernel_user_test]
[ 143.032466] do_one_initcall+0x70/0x1b4
[ 143.038282] do_init_module+0x58/0x1e8
[ 143.039354] load_module+0x181c/0x1920
[ 143.040919] __do_sys_finit_module+0xb8/0x10c
[ 143.041558] __arm64_sys_finit_module+0x20/0x2c
[ 143.044052] invoke_syscall+0x44/0x104
[ 143.044663] el0_svc_common.constprop.0+0x44/0xec
[ 143.045562] do_el0_svc+0x38/0x98
[ 143.047935] el0_svc+0x2c/0x84
[ 143.048175] el0t_64_sync_handler+0xb8/0xbc
[ 143.048295] el0t_64_sync+0x190/0x194
[ 143.049274] Code: f9000006 f81f80a7 d65f03c0 361000c2 (b9400026)
[ 143.050933] ---[ end trace 0000000000000000 ]---
Segmentation fault

root@qemu-ubuntu:~# modprobe memdup_kernel_user_test test_case=1
[ 87.896982] test case 1: kmemdup failed, PTR_ERR(ptr) = 16
modprobe: ERROR: could not insert 'memdup_kernel_user_test': Invalid argument

root@qemu-ubuntu:~# modprobe memdup_kernel_user_test test_case=2
[ 124.032509] test case 2: memdup_user failed, PTR_ERR(ptr) = -14
modprobe: ERROR: could not insert 'memdup_kernel_user_test': Bad address

root@qemu-ubuntu:~# modprobe memdup_kernel_user_test test_case=3
[ 155.496285] test case 3: memdup_user failed, PTR_ERR(ptr) = 16
modprobe: ERROR: could not insert 'memdup_kernel_user_test': Invalid argument

To sum it up, it is:
1) If call kmemdup() with the src == NULL, a NULL pointer dereference
fault happened.
2) If call kmemdup() with the len == 0, an invalid address value
ZERO_SIZE_PTR returned, consider that many existing code check
kmemdup() return value like this:
ptr = kmemdup();
if (!ptr) {
/* allocation failed */
}
this could be a problem, but no fault happended, memcpy() will do
nothing if copy length is zero, my previous statement is wrong.
3) If call memdup_user() with src == NULL, -EFAULT returned. Because
copy_from_user() takes care of the NULL pointer case, there is no
fault to happend.
4) If call memdup_user() with len == 0, an invalid address value
ZERO_SIZE_PTR returned. The existing code uses IS_ERR() to check
memdup_user() return value, unfortunately, the check range of the
macro function doesn't contain ZERO_SIZE_PTR value.

For 1), (2), we can add the following code to kmemdup() to eliminate:
if (!src || len == 0)
return NULL;

For 4), we can change the statement if (!p) of memdup_user() to
if (ZERO_OR_NULL_PTR(s)) to solve that.

BTW, the return values of kmemdup() and memdup_user() got a little
bit confused for now:
. kmemdup() can return ZERO_SIZE_PTR, NULL, and a valid memory allocation
address, the caller should check those return values with ZERO_OR_NULL_PTR(),
but many existing code don't follow this.
. memdup_user() can return ZERO_SIZE_PTR,-ENOMEM,-EFAULT,NULL, and a valid
memory allocation address, the caller should check those return values with
ZERO_OR_NULL_PTR() and IS_ERR() at the same time, but i can't find any code
do things like this.

> > Signed-off-by: Xujun Leng <lengxujun2007@xxxxxxx>
> > ---
> > mm/util.c | 3 +++
> > 1 file changed, 3 insertions(+)
> >
> > diff --git a/mm/util.c b/mm/util.c
> > index dd12b9531ac4..d1a3b3d2988e 100644
> > --- a/mm/util.c
> > +++ b/mm/util.c
> > @@ -128,6 +128,9 @@ void *kmemdup(const void *src, size_t len, gfp_t gfp)
> > {
> > void *p;
> >
> > + if (!src || len == 0)
> > + return NULL;
> > +
> > p = kmalloc_track_caller(len, gfp);
> > if (p)
> > memcpy(p, src, len);

> Why should we take care of kmemdup(), but not memdup_user() ? Shouldn't
> it suffer from similar problems?
By the foregoing, i think that both kmemdup() and memdup_user() need to
change.
--

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