Message-Id: <20230309100415.2382-1-lengxujun2007@126.com>
Date:   Thu,  9 Mar 2023 18:04:15 +0800
From:   Xujun Leng <lengxujun2007@....com>
To:     david@...hat.com
Cc:     akpm@...ux-foundation.org, lengxujun2007@....com,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH] mm: fix potential invalid pointer dereference in kmemdup()

> On 09.03.23 07:46, Xujun Leng wrote:
>>> 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@...u-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@...u-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@...u-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@...u-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@....com>
>>>> ---
>>>>    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.
>
> The issue is that you can call mostly any kernel function with 
> unsupported arguments and trigger crashes. It all depends on with which 
> parameters functions are expected to be called.
>
> If kmemdup() is not expected to be called with !src or !len, all is 
> fine. And if there are no broken cases, existing code obeys these rules.
>
> Of course, we could improve the documentation or adjust the 
> implementations, if there is real need to.
>
> But adjusting individual functions here while others are left with he 
> same, theoretical (!) problems, is not a good approach IMHO.

Yes, you're right. The best way is to change kmalloc_slab(), let it always
return NULL on allocate failure, even for requested size == 0. And of course,
the detailed error message ZERO_SIZE_PTR will lost.

On the other hand, except kmemdup() and memdup_user(), the other functions
in mm/util.c, who called kmalloc_track_caller(), like kstrdup(), kstrndup(),
kmemdup_nul(), memdup_user_nul(), their all do argument check, and the 
len >= 1(if ignore the wrap case). So if we need change kmemdup() and
memdup_user() a little, to let those all functions keep the same? If the 
answer is NO, we can end the disscuss to save your time.
--
Thanks,

Xujun Leng

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