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Add seal if ashmem-dev is backed by memfd 

Need to seal the buffer size in align with ashmem if set to PROT_READ
only to prevent untrusted remote process to shrink the buffer size and
crash it.

Bug: 294609150
Test: build
Ignore-AOSP-First: Security
Change-Id: I9288cf30b41e84ad8d3247c204e20482912bff69
This commit is contained in:
Keith Mok 2023-08-31 00:31:35 +00:00
parent 711d96e4fb
commit f83c5c8fec
1 changed files with 25 additions and 4 deletions
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29
libcutils/ashmem-dev.cpp
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@ -349,6 +349,12 @@ static int memfd_create_region(const char* name, size_t size) {
return -1; return -1;
} }
// forbid size changes to match ashmem behaviour
if (fcntl(fd, F_ADD_SEALS, F_SEAL_GROW | F_SEAL_SHRINK) == -1) {
ALOGE("memfd_create(%s, %zd) F_ADD_SEALS failed: %m", name, size);
return -1;
}
if (debug_log) { if (debug_log) {
ALOGE("memfd_create(%s, %zd) success. fd=%d\n", name, size, fd.get()); ALOGE("memfd_create(%s, %zd) success. fd=%d\n", name, size, fd.get());
} }
@ -400,14 +406,29 @@ error:
} }
static int memfd_set_prot_region(int fd, int prot) { static int memfd_set_prot_region(int fd, int prot) {
/* Only proceed if an fd needs to be write-protected */ int seals = fcntl(fd, F_GET_SEALS);
if (seals == -1) {
ALOGE("memfd_set_prot_region(%d, %d): F_GET_SEALS failed: %s\n", fd, prot, strerror(errno));
return -1;
}
if (prot & PROT_WRITE) { if (prot & PROT_WRITE) {
/* Now we want the buffer to be read-write, let's check if the buffer
* has been previously marked as read-only before, if so return error
*/
if (seals & F_SEAL_FUTURE_WRITE) {
ALOGE("memfd_set_prot_region(%d, %d): region is write protected\n", fd, prot);
errno = EINVAL; // inline with ashmem error code, if already in
// read-only mode
return -1;
}
return 0; return 0;
} }
if (fcntl(fd, F_ADD_SEALS, F_SEAL_FUTURE_WRITE) == -1) { /* We would only allow read-only for any future file operations */
ALOGE("memfd_set_prot_region(%d, %d): F_SEAL_FUTURE_WRITE seal failed: %s\n", fd, prot, if (fcntl(fd, F_ADD_SEALS, F_SEAL_FUTURE_WRITE | F_SEAL_SEAL) == -1) {
strerror(errno)); ALOGE("memfd_set_prot_region(%d, %d): F_SEAL_FUTURE_WRITE | F_SEAL_SEAL seal failed: %s\n",
fd, prot, strerror(errno));
return -1; return -1;
} }