Thanks Dave for reviewing.
On 21/02/2022 21:44, Dr. David Alan Gilbert wrote: > * Dov Murik (dovmu...@linux.ibm.com) wrote: >> When pc_system_parse_ovmf_flash() parses the optional GUIDed table in >> the end of the OVMF flash memory area, the table length field is checked >> for sizes that are too small, but doesn't error on sizes that are too >> big (bigger than the flash content itself). >> >> Add a check for maximal size of the OVMF table, and add an error report >> in case the size is invalid. In such a case, an error like this will be >> displayed during launch: >> >> qemu-system-x86_64: OVMF table has invalid size 4047 >> >> and the table parsing is skipped. >> >> Signed-off-by: Dov Murik <dovmu...@linux.ibm.com> >> >> --- >> >> v2: >> - add error message example to commit description >> - replace magic numbers 48 and 50 with size calculations (thanks Phil MD) >> --- >> hw/i386/pc_sysfw_ovmf.c | 20 ++++++++++++++++---- >> 1 file changed, 16 insertions(+), 4 deletions(-) >> >> diff --git a/hw/i386/pc_sysfw_ovmf.c b/hw/i386/pc_sysfw_ovmf.c >> index f4dd92c588..1c9a16e9e6 100644 >> --- a/hw/i386/pc_sysfw_ovmf.c >> +++ b/hw/i386/pc_sysfw_ovmf.c >> @@ -24,11 +24,14 @@ >> */ >> >> #include "qemu/osdep.h" >> +#include "qemu/error-report.h" >> #include "hw/i386/pc.h" >> #include "cpu.h" >> >> #define OVMF_TABLE_FOOTER_GUID "96b582de-1fb2-45f7-baea-a366c55a082d" >> >> +static const int bytes_after_table_footer = 32; >> + >> static bool ovmf_flash_parsed; >> static uint8_t *ovmf_table; >> static int ovmf_table_len; >> @@ -38,6 +41,8 @@ void pc_system_parse_ovmf_flash(uint8_t *flash_ptr, size_t >> flash_size) >> uint8_t *ptr; >> QemuUUID guid; >> int tot_len; >> + int max_tot_len = flash_size - (bytes_after_table_footer + >> + sizeof(guid) + sizeof(uint16_t)); >> >> /* should only be called once */ >> if (ovmf_flash_parsed) { >> @@ -52,12 +57,13 @@ void pc_system_parse_ovmf_flash(uint8_t *flash_ptr, >> size_t flash_size) >> >> /* >> * if this is OVMF there will be a table footer >> - * guid 48 bytes before the end of the flash file. If it's >> - * not found, silently abort the flash parsing. >> + * guid 48 bytes before the end of the flash file >> + * (= 32 bytes after the table + 16 bytes the GUID itself). >> + * If it's not found, silently abort the flash parsing. >> */ >> qemu_uuid_parse(OVMF_TABLE_FOOTER_GUID, &guid); >> guid = qemu_uuid_bswap(guid); /* guids are LE */ >> - ptr = flash_ptr + flash_size - 48; >> + ptr = flash_ptr + flash_size - (bytes_after_table_footer + >> sizeof(guid)); >> if (!qemu_uuid_is_equal((QemuUUID *)ptr, &guid)) { >> return; >> } I'll probably split the two hunks above (without max_tot_len) to a separate patch "hw/i386: Replace magic number with field length calculation". >> @@ -66,7 +72,13 @@ void pc_system_parse_ovmf_flash(uint8_t *flash_ptr, >> size_t flash_size) >> ptr -= sizeof(uint16_t); >> tot_len = le16_to_cpu(*(uint16_t *)ptr) - sizeof(guid) - >> sizeof(uint16_t); > > Instead of the max_tot_len calculation above, is it actually: > max_tot_len = ptr - flash_ptr; > > I think that works out the same and avoids doing the calculation in two > places; it's also logically what you have - you can't read over the > structure you just read. Good call, it indeed gives the same result. I'll change the condition below to: if (tot_len < 0 || tot_len > (ptr - flash_ptr)) and remove the max_tot_len variable, and put this in a separate patch that only adds this condition to solve the overflow problem. Thanks, -Dov > > Dave > >> - if (tot_len <= 0) { >> + if (tot_len < 0 || tot_len > max_tot_len) { >> + error_report("OVMF table has invalid size %d", tot_len); >> + return; >> + } >> + >> + if (tot_len == 0) { >> + /* no entries in the OVMF table */ >> return; >> } >> >> >> base-commit: 48033ad678ae2def43bf0d543a2c4c3d2a93feaf >> -- >> 2.25.1 >>