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kernel: sucompat: increase reliability, commonize and micro-optimize tiann #2656

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On plain ARMv8.0 devices (A53,A57,A73), strncpy_from_user_nofault() sometimes
fails to copy `filename_user` string correctly. This breaks su ofc, breaking
some apps like Termux (Play Store ver), ZArchiver and Root Explorer.

Apply the susfs patch

This does NOT seem to affect newer ARMv8.2+ CPUs (A75/A76 and newer)

My speculation? ARMv8.0 has weak speculation :)

here we replace `ksu_strncpy_from_user_nofault` with ksu_strncpy_from_user_retry:
- ksu_strncpy_from_user_nofault as fast-path copy
- fallback to access_ok to validate the pointer + strncpy_from_user
- manual null-termination just in case, as strncpy_from_user_nofault also does it
- remove that memset, seems useless as it is an strncpy, not strncat

basically, we retry on pagefualt

for usercopies, its not like were doing
    memset(dest, 0, sizeof(dest));
    strncat(dest, var, bytes);

that memset seems unneeded. instead we use strncpy itself to do proper
error and oob check and null term it after.

as for optimizations
- just return early if unauthorized
- commonized logic
- reduced duplication

Tested on:
- ARMv8.0 A73.a53, A57.a53, A53.a53
- ARMv8.2 A76.a55

Stale: tiann #2656

Co-authored-by: backslashxx <118538522+backslashxx@users.noreply.github.com>
Co-authored-by: ShirkNeko <109797057+ShirkNeko@users.noreply.github.com>
Co-authored-by: rsuntk <rsuntk@yukiprjkt.my.id>
Signed-off-by: ShirkNeko <109797057+ShirkNeko@users.noreply.github.com>
This commit is contained in:
ShirkNeko
2025-09-25 19:41:26 +08:00
parent a85ce2bc35
commit dd1eb98963
5 changed files with 84 additions and 223 deletions
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91
kernel/kernel_compat.c
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@@ -145,80 +145,12 @@ ssize_t ksu_kernel_write_compat(struct file *p, const void *buf, size_t count,
#endif
}
#if LINUX_VERSION_CODE >= KERNEL_VERSION(5, 8, 0) || defined(KSU_OPTIONAL_STRNCPY)
long ksu_strncpy_from_user_nofault(char *dst, const void __user *unsafe_addr,
long count)
{
return strncpy_from_user_nofault(dst, unsafe_addr, count);
}
#elif LINUX_VERSION_CODE >= KERNEL_VERSION(5, 3, 0)
long ksu_strncpy_from_user_nofault(char *dst, const void __user *unsafe_addr,
long count)
{
return strncpy_from_unsafe_user(dst, unsafe_addr, count);
}
#else
// Copied from: https://elixir.bootlin.com/linux/v4.9.337/source/mm/maccess.c#L201
long ksu_strncpy_from_user_nofault(char *dst, const void __user *unsafe_addr,
long count)
{
mm_segment_t old_fs = get_fs();
long ret;
if (unlikely(count <= 0))
return 0;
set_fs(USER_DS);
pagefault_disable();
ret = strncpy_from_user(dst, unsafe_addr, count);
pagefault_enable();
set_fs(old_fs);
if (ret >= count) {
ret = count;
dst[ret - 1] = '\0';
} else if (ret > 0) {
ret++;
}
return ret;
}
#endif
long ksu_strncpy_from_user_retry(char *dst, const void __user *unsafe_addr,
long count)
{
long ret;
ret = ksu_strncpy_from_user_nofault(dst, unsafe_addr, count);
if (likely(ret >= 0))
return ret;
// we faulted! fallback to slow path
if (unlikely(!ksu_access_ok(unsafe_addr, count))) {
#ifdef CONFIG_KSU_DEBUG
pr_err("%s: faulted!\n", __func__);
#endif
return -EFAULT;
}
// why we don't do like how strncpy_from_user_nofault?
ret = strncpy_from_user(dst, unsafe_addr, count);
if (ret >= count) {
ret = count;
dst[ret - 1] = '\0';
} else if (likely(ret >= 0)) {
ret++;
}
return ret;
}
long ksu_copy_from_user_nofault(void *dst, const void __user *src, size_t size)
{
#if LINUX_VERSION_CODE >= KERNEL_VERSION(5, 8, 0)
return copy_from_user_nofault(dst, src, size);
#elif LINUX_VERSION_CODE >= KERNEL_VERSION(5, 3, 0)
return probe_user_read(dst, src, size);
#else
// https://elixir.bootlin.com/linux/v5.8/source/mm/maccess.c#L205
long ret = -EFAULT;
@@ -238,3 +170,22 @@ long ksu_copy_from_user_nofault(void *dst, const void __user *src, size_t size)
return 0;
#endif
}
/*
* ksu_copy_from_user_retry
* try nofault copy first, if it fails, try with plain
* paramters are the same as copy_from_user
* 0 = success
* + hot since this is reused on sucompat
*/
__attribute__((hot))
long ksu_copy_from_user_retry(void *to,
const void __user *from, unsigned long count)
{
long ret = ksu_copy_from_user_nofault(to, from, count);
if (likely(!ret))
return ret;
// we faulted! fallback to slow path
return copy_from_user(to, from, count);
}